Melissa, my post card was also larger than the minimum size, but smaller than the max size that the PO said. I think it will still go through with post card postage, although I would probably just put a regular 1st class stamp on it rather than going out and buying more.
LeighAnn, I really like your themed entry! All the little letter and post box stamps are really cute. I certainly will accept it to this challenge, but it would be a fun experiment to see if those sequins go through the mail. I would also think you would really need to use strong adhesive if you are going to have layers (which I did say I would accept for the challenge). I didn't see anything about embellishments or stickers being prohibited, just a max thickness, from the PO guidelines, but not sure about how it would work in their machines...
Anyhow, super cute and thanks for playing. I hope some of you might do a second entry (or a first). I have recently just been having fun doing something a little different. Even if you later smack it on a card or in an envelope, it helps me stretch my creativity.