I think mine are doing the same. I bought some shiny large letter envelopes and I was worried the stamps might flake off when they dry... I used a few old gum stamps I found in my boxes rather than the stickers.... and then I receive this. So was trying to determine which letter it was.

I think my P.O is a bit slack with cancelling stamps, so theres no overlap of stamp to tell if there was a postage stamp there or not I guess.

Ill just tape the little buggers on 🙂