I've got Adventure Bundle which should include 'a 10% discount off standard pricing for all future digital Dungeons & Dragons adventure book purchases on D&D Beyond!'

Now I'm looking at 'Fizban's Treasury of Dragons' & it gives me full price at the checkout. Is this an issue?

Fizban's Treasury of Dragons isn't an adventure book, it's a sourcebook. As such, it doesn't qualify for the adventure bundle discount

Oh, I got you. I haven't figured out the difference. Thank you for reply :)

If you go into the marketplace, there are categories on the right for sourcebooks and for adventures