Different postage prices apply to postcards, letters, large envelopes (flats), and parcels. In general, the bigger your mailpiece, the more it will cost to mail.

What size envelope needs extra postage?

Pay close attention to envelopes such as greeting cards marked “Extra Postage Required.” In most cases, a letter-size mailpiece is classified and priced as a large envelope (flat) if it exceeds 3.5 ounces or 6-1/8" by 11-1/2" by 1/4" thick.

Does envelope size matter for postage?

YES. According to USPS.com, for an envelope to be eligible for mailing at the price for letters, a piece must be: Rectangular. At least 3-1/2 inches high x 5 inches long x 0.007 inch thick.