What happens if I mail an oversized postcard with a postcard stamp?

Using a postcard stamp on an oversized card means you have underpaid the postage, and USPS handles that in one of a few ways depending on your card.

• With a return address: it comes back to you marked "postage due."
• Without a return address: it may be delivered with postage due charged to the recipient, or it may not be delivered at all.
• Either outcome delays or derails your mailing.

The fix is simple: match your postage to your postcard size. Anything larger than the standard postcard dimensions needs letter-rate postage.