Basically, I was trying to set up a guild as benefactors for my players. I mostly run pre-made campaigns, but two of them linked together well (Legend of the Ice Lady and Dragon of Icespire peak, if anyone's familiar. Party travels through the location of Ice Lady to reach the location of Dragon and the big reward from Ice Lady will be useful fighting a white dragon). So, I had one of the guild members send a letter to the party about the dragon, offering to buy any harvested material they get from it off them, as a way to both kick off the two campaigns and introduce the guild (the party had encountered the member before as the shopkeeper of a traveling magic shop who bought some other stuff off them).

The guild is a sort-of jack-of-all-trades group that the party had never heard of before as it's fairly small, though with fairly strong members. First thing my party does after being offered what is basically freelance work from a member of unknown rank and authority? Decide they're in the guild now, use prestidigitation to add the guild mark to the side of their cart, and (when a letter about fines/membership fees was dropped on the spellcaster's head from an unseen source) burn the letter.

So, my question is basically what consequences can this have without completely derailing my campaign. I've already got a scout following them who is very unhappy with them. (The scout was supposed to assess them for potential membership. Needless to say that's not going to happen now.) I'm considering having the guild now act as a hostile force, except they're supposed to be a good guild and the emblem was only displayed for like an hour, albeit in a crowded location where plenty of people saw. I also considered having enemies of the guild attack the group, except it's a fairly small, not very well known guild, so they don't really have much in the way of enemies. Also have a high-ranked guild member made who can encounter and take reparations from the party by force, but I'd like some other ideas if possible.

For clarity the following is my opinion, I use commanding language as a habit, see it as advice not instruction.

What you are experiencing is emergent storytelling, which is a good thing.  The players took it upon themselves to establish their own ambitions and execute their own plans. What do you do with that?  Make it the focus of the story.

Generally speaking you want to make sure that whatever decisions players make, whatever direction they take the campaign, that it feels to them like this was the plan all along.  As if to say "I knew you were going to do that".

So what you don't want to do is punish their behavior, which is not to say that there are no consequences, but there is a fine line between creating consequences for the characters for their actions and punishing the players for their actions.  That line is so thin, it can be really easy to screw it up.  The main thing is that consequences create great narrative twists, and punishments, teach the players what not to do in your game and what draws the DM's Irie. 

My way of handling it would be to draw it out.

First, I would let them dig this hole a little deeper, even tempt them with opportunities.  After all, they are representing a guild, that likely commands some respect, advantages, some perks.  Like, the scout follows them, reports on them, but doesn't do more.  Why would he, he's a scout, it's not his job to police people.  I would let the players continue to use "the guild" as a way to get benefits, and advantages as such a guild member might get.  I would let them think they are clever, that they are getting away with it for a while, like quite a while, until the players believe it themselves.  Right when using the guilds name becomes routine and they think they pulled it off.. that's when consequences come calling.

As for what the consequences are, that really depends on how the guild operates.  A mercenary guild would send thugs, a thieves guild would send assassins, a merchant guild would send the authorities.  You have to kind of decide how the guild thinks, do they take the law into their own hands, or do they bring down the law.  Whatever it is however, I would try to isolate the players, put them on the hot seat individually.  Im guessing this whole "idea" wasn't some group consciousness thing, there is a culprit behind the whole thing, this whole plan is someones brainchild and I would try to get these guys to turn on each other a bit, do some finger pointing and give them a chance to let one of them be a fall guy for the whole thing... 

Of course... thats just what I would do.

I'd largely agree with the above. 

I'd add that this feels like one of those player-originated hooks, which are awesome. See if you can slowly escalate it. Turn it into a story about how these adventurer's deal with the consequences and, ideally, the stakes will keep getting upped. Maybe the finale is that they take over the Guild.

I say slowly, it's important that the party feels slighted by the Guild, not oppressed. You want to "annoy"* them and antagonise them*, not punish them as OSR says.

* The PCs, not the players, obviously.

It would take some DM skill to pull this off properly.

The guild might appeal to the local lord to enforce the fines against the party, forcing them to pay, OR if you want to take this as a cool DMing opportunity, you can have the guild offer to waive the fines in exchange for services, and have the guild essentially send the party on a mission to clear their debt. Maybe bandits or monsters are attacking guild shipments to a certain area, or maybe one of their members has gone missing and they want you to investigate the last known location, or whatever other mission you have floating around without a story tie-in.

But also be prepared for the players to also evade the guards, so think about ways to incentivize them not to; maybe important NPC's won't talk to them until they clear up this "misunderstanding with the guard", and maybe local services are denied to them because of their reputation as liars who don't pay their debts. 

Quote from CharlesThePlant >>

The guild might appeal to the local lord to enforce the fines against the party, forcing them to pay, OR if you want to take this as a cool DMing opportunity, you can have the guild offer to waive the fines in exchange for services, and have the guild essentially send the party on a mission to clear their debt. Maybe bandits or monsters are attacking guild shipments to a certain area, or maybe one of their members has gone missing and they want you to investigate the last known location, or whatever other mission you have floating around without a story tie-in.

But also be prepared for the players to also evade the guards, so think about ways to incentivize them not to; maybe important NPC's won't talk to them until they clear up this "misunderstanding with the guard", and maybe local services are denied to them because of their reputation as liars who don't pay their debts. 

 exactly what I was going to say.

The guild is small but local, and the townsfolk are quite well aware as to who is or isn't part of the local chapter. The town may also be upset with "inferior quality of service" than what they're used to. The shopkeepers stop wanting to sell to them (or will at significantly marked up prices) and next, the guards are starting to get involved, and if they try to run they run smack into a couple of SIGNIFICANTLY higher level NPCs that are big fans of the guild (the guild came in clutch and helped these rougher looking guys out) that offer them a choice. Broken kneecaps and other body parts (and possibly  "polymorphed" into something very very unpleasant) before the guards "happen" to find them, or they can do some favors for the guild. 

...Yeah... so as I said in the initial post, I mostly (read only) run pre-made adventures. I'm terrible at improvising and definitely can't make an original adventure. I'm probably not really cut out to DM.

I guess I can try having rumors spread about them. The guild is really small and also the spread-thin main defense for most people, considering the local lord doesn't even care about a dragon attacking a settlement so I don't think he'll do anything either (that might actually work for a plot hook if I ever get up the courage to try making my own adventure). Limiting services could work. Not where they are right now since they're dealing with some attacks in a small village that is very grateful, and the next area their going to is under dragon attack, though I'm considering having the guild deal with the dragon instead.

Ok, guess I'll do that. I've got a month to work out the details so I can probably manage. Thanks for all the help.

If you want to use it as a plot hook but don't want to improvise an adventure, you could lead them into one of the dungeons from Tales From the Yawning Portal or something, tell them the guild will forgive them if they explore it for them or something. 

Hit 'em where it hurts! Their wallets. No merchant will sell to or buy from them until the fines and membership fees are paid. It should be a relatively small fine since it was only one offense. Then some interest since it's taken them too long to pay. 

They go to the blacksmith to get a new weapon? There's a "wanted" poster there. "Sorry, lassie. I no can take yer monies 'til whatever this is is cleared up!"

They want to buy some health potions? Another poster. "Love to sell to you, no doubt, but rules are rules. I'm not losing my business license for complete strangers."

Or the guild could just have them pickpocketed or maybe a Geas cast on them. But since this seems to be a minor offense I doubt they'd go that far. 

Broadly speaking, the PCs are engaged in a type of fraud, and the details of possible consequences depend extensively on the society.

The easiest option to play out is that it's illegal and subject to a civic fine. This means the PCs will either have to pay their fine or deal with the black market (which will generally have higher prices). If it's not illegal, the guild will have to do its own enforcement, which has historically been done by a lot of means, most of which either won't be very effective on PCs or will have disruptive effects.

Quote from LamiaViper>>

...Yeah... so as I said in the initial post, I mostly (read only) run pre-made adventures. I'm terrible at improvising and definitely can't make an original adventure. I'm probably not really cut out to DM.

I guess I can try having rumors spread about them. The guild is really small and also the spread-thin main defense for most people, considering the local lord doesn't even care about a dragon attacking a settlement so I don't think he'll do anything either (that might actually work for a plot hook if I ever get up the courage to try making my own adventure). Limiting services could work. Not where they are right now since they're dealing with some attacks in a small village that is very grateful, and the next area their going to is under dragon attack, though I'm considering having the guild deal with the dragon instead.

 You don't need to turn this into the main story, just run with it. As you can see, there are plenty of players here that can help you with ideas if you need them. This could simply be a supplementary story, a collection of side quests that tell a B-story. You'll be surprised just how much you can do just by running with them. Remember, they're going to be creating their own story as well. Usually, I find players end up kicking up threads for me to tug on. They already have - they cheesed off a guild, and now you're tugging on that thread by asking about consequences.

You've got the skills in you to pull this off. Just keep pulling at those threads.

 A secret guild would not put out the word against them. Neither would a small guild. no power to enforce it.

They would decide if the offense was enough to warrant a fine or punishment first. If the party only got a discount on a few cheep items then the discount plus "processing fees"( cost of the thief to get it back) would be enough.
If the guild thought they brought a bad reputation to the guild the fines would be quite high. All guilds live by their reputation.

in the end the guild could stay in the campaign and still send work their way, or work against them. Or just be part of the background color.
They could publicly offer jobs the group would like to get but would need to atone with the guild first to get.

I like the "solidarity angle among the different guilds" angle.  You can make a side a challenge out of this.  While they haven't smooth things over with the guild they offended, the characters now run into challenges with doing business with the other guilds. That means they have to engage in deceptive tactics to hide their identify when negotiating for work or trade, seek out and identify employers who work off book, run out of town if they are discovered or pay a heavy fine (or "fine) to a local magistrates office for violating codes of business conduct.

And you don't need to plan this out.  Us examples in DM guide and tutorials to just make a table and then occasional role on it.  Doesn't have to be every session either.  But this will keep you players on their toes and knowing that they plans can go sideways at any moment. 

Eventually give them a quest to smooth this over and move on, but for now have fun with it the randomness and chaos it can cause. 

Soooo...

Officially, GUilds are like Unions, except they all have some sort of Noble support or backing that gives the guild the baseline authority to act and exist.

Well, historically speaking. 

Claiming to be that which you are not was often deeply criminalized (that supported the power of the nobility and made sure taxes are paid) but more importantly, it would lead to outlawry.

Being declared an outlaw means that anyone who helps you -- let's you camp on their land or even eat their food -- is now a criminal as well.

Before you get to outlawry, of course, you have several attempts to get those involved to come clean, additional fines, and the slow and eroding trust of everyone (especially other guilds and merchants, who would stop doing business with them).

What does that look like? Well, staggering back from a dungeon and the Inn turns you away. Need supplies?  Well, every town in the surrounding 100 miles won't sell you anything if they recognize you. And you will have to prove you aren't them, more often than proving you are since strangers will be assumed bad actors.

Need healing? Nope. Need potions, or spells or just a place to camp out? Nope -- unless you shack up with shady characters who probably have a problem with the local nobility and aren't very popular among the townsfolk -- but you do that and you might as well admit to being ruffians and worthy of banditry.

Guilds did not and would not mess around. They are not direct actors, and they would operate on the basis of reputation -- which means the PCs could get into a war over reputation, but there isn't much adventuring or sword fighting there. And nothing says love like a Guild hired assassin being sent after you in the night.