If you send someone something with Galder's Speedy Courier, can they place something in the chest to be sent back (so long as they don't trigger the spell to end i.e. by removing all the items)? 

I would argue that the spell does not specifically state that the receiver cannot place items into the chest. Also, I would argue that since the spell has a rather hefty 25gp cost for a low level spell, that it would be fair for them to be able to place items in it too. Since they still only have the remainder of the 10mins to do so, it would still have its limits.

I would be willing to let the recipient put items inside - but RAW the only person who is explicitly called out as being able to put items inside is the caster - so I would say no - from a strict reading - the recipient can only take items out.

Quote from Thundergun105 >>

If you send someone something with Galder's Speedy Courier, can they place something in the chest to be sent back (so long as they don't trigger the spell to end i.e. by removing all the items)? 

I would argue that the spell does not specifically state that the receiver cannot place items into the chest. Also, I would argue that since the spell has a rather hefty 25gp cost for a low level spell, that it would be fair for them to be able to place items in it too. Since they still only have the remainder of the 10mins to do so, it would still have its limits.

"When the elemental disappears, any items not taken from the chest reappear on the ground at your feet." I'd rule that as any items not taken reappear on the ground at the feet of the person who put the items in the chest. So the target could put things in the chest if they wanted, they just wouldn't go anywhere.

It's a magical courier, as in FedEx. You can either accept delivery or refuse it; you can't hand the courier a different item to bring back. That would require a new waybill casting.

When interpreting RAW, the design intent is that spells and effects only do what they say they do, not what they don't say they don't do. As such my understanding of RAW is the same as the answers above.

However, in my own game I'd interpret it more loosely and rule the same way you and Emmber suggest.

Quote from AntonSirius >>

"When the elemental disappears, any items not taken from the chest reappear on the ground at your feet." I'd rule that as any items not taken reappear on the ground at the feet of the person who put the items in the chest. So the target could put things in the chest if they wanted, they just wouldn't go anywhere.

It's a magical courier, as in FedEx. You can either accept delivery or refuse it; you can't hand the courier a different item to bring back. That would require a new waybill casting.

In the context of the spell, your is you as in the caster, not you as in the putter - the spell consistently uses "you" to refer to the caster throughout its spell text.  The question is whether or not the recipient can put items in the chest, not where items in the chest go when the elemental disappears.

The spell magically stops non-recipients from taking things from the chest, which means it wouldn't be weird for the spell to stop non-senders from putting things in the chest, but it doesn't do that.  When the caster casts the spell, anyone can put things in the chest - the text that guarantees only recipients can take is conspicuously absent from the text block about putting things into the chest.  I don't see any text anywhere in the spell stopping the recipient from putting things in the chest, just as anyone else can.

tl;dr Yes, OP can put items in the chest to return to sender, because spells only do what they say they do, and the spell doesn't say it stops them from doing so.  It does say items in the chest are returned to the sender.

I think AntonSirius' quoted text is important. Even if the recipient could put items inside - only items that the sender put in - that weren't taken out - are returned to the sender. Anything else would be left behind.

I find it unusually specific to word it that way. It could just say "any remaining items inside the chest reappear on the ground at your feet" - which would include everything in the chest - but instead they opted to specifically call out only items that were not taken out. That's very weirdly specific.

If you're a higher level Chronurgist wizard you could use the Arcane Abeyance and another casting to put the mote in the box too, which the recipient can use to send new things to you. 

Personally, I'd allow the caster to choose who is recipient and who is sender. It would take two castings for a full exchange, which seems reasonable and it increases flexibility. That way the spell does what seems intended: allow people to trade small items remotely - such as those who prefer to stay in their tower (like Galder's tower -- there was an intended theme here). 

Quote from quindraco >>

the spell consistently uses "you" to refer to the caster throughout its spell text.

 

Quote from quindraco >>

When the caster casts the spell, anyone can put things in the chest

Nope. "While the spell lasts, you can deposit as many items inside the chest as will fit."

If 'you' only ever refers to the caster, then the text is very explicit about who can put things in the chest.

It definitely seems to be intended as a 1 way courier, and the wording doesn't indicate any way of cheesing around that.

If it gets to be an issue just put a bag of holding in there and stow and retrieve any items in that. 

Can you put small creatures in the chest?

items are not creatures (there are items that can turn into creatures, though), but in general, no.

But DM might let you use bag of holding/portable hole as a workaround. Creatures can be put into these and the bag/hole put into the chest. Technically the creatures are in a different dimension so only the items are actually in the chest.