I once mailed a card taped to a 3x5 card once to a friend as a joke and it made it just fine. Tried mailing just the card in a toplader with a stamp on it but the PO said it was too small hence the 3x5 card.
Andrew- If I can switch to the Rockies for this break that would be great. This is like the 3rd break that they barely have anything. If not no big deal.