Yes, you can use postcard stamps on a letter, as long as the stamps on the envelope add up to at least the First-Class letter rate. A postcard stamp is worth $0.61, and a standard one-ounce letter needs $0.78 in postage, so a single postcard stamp by itself falls $0.17 short.1United States Postal Service. Notice 123 – January 18, 2026 Price Change Add more stamps to cover the gap and the letter goes through.
Postcard stamps are printed with the word “Postcard” instead of a dollar amount, and like Forever stamps they always carry the current postcard rate. If you bought them years ago at a lower price, they’re still worth $0.61 today. That’s the value they contribute when you stick one on a letter.2USPS. Postage Stamps – The Basics USPS doesn’t care what mix of stamps you use, only that the total face value meets the required postage.
How to Cover the $0.17 Difference
USPS sells low-denomination “additional postage” stamps in 1¢, 2¢, 3¢, 4¢, 5¢, and 10¢ values at post offices and through the Postal Store online.3USPS. How to Send a Letter or Postcard – Section: Step 3: Calculate Postage With a $0.61 postcard stamp already on the envelope, a few combinations work:
- A 10¢ stamp plus a 5¢ stamp plus a 2¢ stamp gets you to exactly $0.78.
- A 10¢ stamp plus two 4¢ stamps gets you to $0.79, overpaying by a penny.
- Two postcard stamps total $1.22, which covers the letter rate with $0.44 to spare.
Overpaying doesn’t cause a problem: the letter still gets delivered, but USPS won’t refund the extra. Two postcard stamps are the easy fallback when you don’t have small-denomination stamps around. Picking up a few penny and nickel stamps at the post office makes topping off postcard stamps painless later.
Placing the Stamps
All postage goes in the upper right corner of the address side. Arrange multiple stamps in a row or a small cluster, and don’t overlap them. A stamp partly hidden behind another one may not be counted toward your postage total.4USPS. Domestic Mail Manual – Postage Stamps A standard business envelope has room for two or three stamps side by side without any trouble.
When $0.78 Isn’t Enough
The $0.78 rate assumes a one-ounce letter in a standard rectangular envelope that postal machines can process. Two situations push the required postage higher.
Weight. Roughly four sheets of printer paper in a business envelope weigh about an ounce.3USPS. How to Send a Letter or Postcard – Section: Step 3: Calculate Postage Each additional ounce adds $0.29, so a two-ounce letter runs $1.07 and a three-ounce letter costs $1.36. First-Class letters cap out at 3.5 ounces.1United States Postal Service. Notice 123 – January 18, 2026 Price Change If you’re unsure, weigh the sealed envelope on a kitchen scale before choosing stamps.
Non-machinable envelopes. Letters that can’t run through sorting machines carry a $0.49 surcharge, which bumps a one-ounce letter to $1.27.5United States Postal Service. Notice 123 – January 18, 2026 Price Change Square greeting-card envelopes are the most common trigger, along with envelopes that are rigid, lumpy, sealed with clasps or string, made of non-paper material, or addressed parallel to the short side.6Postal Explorer. Physical Standards for Retail Letters, Flats, and Parcels Plan for the surcharge if any of that describes your envelope.
What Happens if the Postage Falls Short
Mailing a letter with only a postcard stamp on it leads to one of two outcomes.
With a return address, USPS stamps the envelope “Returned for Additional Postage” and sends it back to you without attempting delivery. You add the missing postage and re-mail it.7USPS. How is Undeliverable and Misdelivered Mail Handled
Without a return address, the letter may be delivered with a “postage due” notice, meaning the recipient pays the shortfall before accepting it. If the recipient refuses or can’t be reached, USPS routes the letter to its Mail Recovery Center, where it may eventually be opened to identify the sender or disposed of.7USPS. How is Undeliverable and Misdelivered Mail Handled Either outcome delays your letter by days or weeks.
The reliable move is simple. Weigh the sealed envelope, confirm the current rate, and add up your stamp values before it leaves your hands.