Stamp, Not a Stamp

I saw these two invitations for Jamie's birthday party by the front door this morning. They were under a bag of trash, and I guessed that is why Jamie forgot to put them in the mailbox. I got them out and was going to make sure they were more visible when I realized that the stamp looked a little funny. Last night Jamie wrote the addresses on the envelopes which I made sure were correct. I told her to put stamps on them. She asked me a question that only makes sense now. She said, "Where are my stamps? I can only find yours."