They aren't stickers, they're vouchers. You can use individual vouchers to build vending machines and other machines. As to the packs, I don't know. I have this question to. I've been earning them a lot lately, and usually they're auto-opened by the system when you don't find them in your inventory (which I looked, and they aren't there.) but i checked the number of vouchers I had before and after earning them and there was no change so they aren't auto-opened either. I don't know where they go or how to find them, but I know what they are!