Hopefully none of you have had this problem.
The USPTO has asked applicants to ensure that they place proper postage on return receipt postcards. An inordinate number have been recently returned to the Patent Office for inadequate postage. Here are post card postage rules:
1. In order to be eligible for the First-Class Mail card rates (currently $0.23 per card, domestic delivery), cards must be of uniform thickness and made of unfolded and uncreased paper or card stock of approximately the quality and weight of a Postal Service stamped card. Cards claimed at the First-Class postcard rate must be:
(a) Rectangular;
(b) No less than 3-1/2 inches high, 5 inches long, and 0.007 inch thick; and
(c) No more than 4-1/4 inches high, 6 inches long, and 0.016 inch thick.2. Cards that measure more than 4-1/4 inches high, 6 inches long, or 0.016 inch thick are charged postage at the First-Class Mail letter rates.
3. Cards that measure less than 3-1/2 inches high, 5 inches long, and 0.007 inch thick are nonmailable. Any return receipt postcard that does not contain sufficient postage or is not acceptable may not be delivered by the United States Postal Service (USPS) to the address provided on the postcard, and, if returned to the Office, may be discarded.
Link.