So, you completed all side-quests, finished the game, partied hard, and was still left with 5 bottlecaps in your backpack that tell you they are to be delivered to ???, along with 4 undeliverable gems.

Well, you didn't miss anything! The bottlecaps were apparently used in the demo and left over in the final game but never got assigned to a quest. That explains why they exist but don't tell you what to do with them.

In the demo the setting is another map, a small clearing in the forest, and the ending of the demo sends you on a quest to Grumblewood Grove to deliver a letter to Greg, so it seems the writing had the demo designed more as a prologue. However, I guess because of time/money constraints the inhabitants in the clearing are just alternative versions of characters from the final game, not independent creatures living someplace else.

So, there the bottlecaps were part of a love quest from alt Irene to alt Donna, Irene asked you to collect them and bring them back, then sent you to deliver them to Donna as a gift. There were actually 6 bottlecaps in the demo, but one was a Discord quest for early players and I don't think is part of the final game. The rest remained in, but since no quest was written for them in the end, they are only part of the collect 40 trinkets achievement.

I wish that, just like the gems, they had at least changed their status to undeliverable since they didn't use them, in order not to confuse us, it was low-hanging fruit as far as fixes go...

Anyway, now you can stop wondering and put the game behind you!

Game spoiler not related to bottlecaps, but also something I saw people wondering about: Yes, you did meet worm-boss, he left the well to some place else. That's why the "well voice" doesn't answer you anymore, he's not there. He climbed to the top of the bamboo forest. It's Jim.

I finished the game having completed everything I could find and Janet told me I did 55 deliveries. If you have less, you probably have something more to do. If you have more, then I missed something myself. Hopefully not!