Mystery solved!

Remember the "I have one extra postcard stamp and therefore must have forgotten to put it on someone's RSVP card but refuse to open all the envelopes to figure out who" saga? No? Well... it was fascinating.

Anyway, the mystery has been solved!

The RSVP card from my aunt... with a different stamp on it!

She's so nice, she wouldn't have minded... I can't wait to tell her that she was the heart of a puzzle! I'm so relieved! :)