Additional postage stamps are the low-denomination stamps (1¢, 2¢, 3¢, 5¢, 10¢, and dedicated ounce stamps) you add to a Forever stamp when your mailpiece costs more to send than the standard one-ounce letter rate. As of January 2026, a Forever stamp covers $0.78 in postage. Anything heavier than one ounce, larger than a standard letter, awkwardly shaped, or headed overseas needs more on top.
When You Need to Add Postage
Four situations trigger the need for extra stamps. Each has its own math.
The Letter Weighs More Than One Ounce
A standard First-Class letter can weigh up to 3.5 ounces, but a single Forever stamp only pays for the first ounce. Each additional ounce costs $0.29.1USPS Postal Explorer. Notice 123 – January 2026 Price Change The 2026 stamped rates:
- 1 ounce or less: $0.78
- 2 ounces: $1.07
- 3 ounces: $1.36
- 3.5 ounces: $1.65
A greeting card with a folded insert, a multi-page letter, or a check tucked into a heavier envelope can push you past an ounce faster than you’d expect. USPS sells a 29¢ additional-ounce stamp, a $1.07 two-ounce stamp, and a $1.36 three-ounce stamp so you don’t have to stack small denominations.2USPS.com. Additional Ounces
The Envelope Is Too Big for Letter Rates
To qualify as a letter, your piece must be rectangular, 3½ to 6⅛ inches tall, 5 to 11½ inches long, and between 0.007 and ¼ inch thick.3Postal Explorer. Sizes for Letters Exceed any of those limits and USPS charges you the large-envelope (flat) rate, which starts at $1.63 for the first ounce and tops out at $5.04 for 13 ounces.1USPS Postal Explorer. Notice 123 – January 2026 Price Change Rigid pieces or ones that aren’t uniformly thick get bumped up to package rates.4USPS. First-Class Mail
The Letter Is Nonmachinable
This is the one people miss. Even a one-ounce letter within standard dimensions gets hit with a $0.49 nonmachinable surcharge if it can’t run through automated sorting equipment.1USPS Postal Explorer. Notice 123 – January 2026 Price Change Common triggers:
- Square envelopes. USPS requires an aspect ratio between 1.3 and 2.5; a square is 1.0, so every square envelope is automatically nonmachinable.
- Rigid contents that don’t flex.
- Lumpy items inside (pens, keys, coins).
- Clasps or string closures that snag machines.
So a one-ounce square wedding invitation costs $0.78 + $0.49 = $1.27.5Postal Explorer. 201 Quick Service Guide Physical Standards for Commercial Letters and Postcards You cover the extra 49¢ with any combination of low-denomination stamps.
The Letter Is Going Abroad
A one-ounce international letter to any country costs $1.70 with a Global Forever stamp. Beyond one ounce, rates climb by destination. A 3.5-ounce letter to Canada is $3.40; the same letter to most European or Asian destinations is $5.75.1USPS Postal Explorer. Notice 123 – January 2026 Price Change The $0.49 nonmachinable surcharge applies internationally too if the shape is irregular. If you’re using regular Forever stamps rather than a Global Forever, you need enough to reach $1.70; three Forever stamps ($2.34) overpays but works.6USPS. First-Class Mail International
The Denominations Available
USPS issues low-value stamps specifically for filling gaps: 1¢, 2¢, 3¢, 5¢, and 10¢.7U.S. Postal Service. USPS Unveils Graceful Low-Denomination Flower Stamps Combine them however you need to hit the exact amount. For common weight scenarios, the dedicated 29¢, $1.07, and $1.36 stamps let you cover the whole piece with one or two stamps instead of a handful.2USPS.com. Additional Ounces
Forever stamps you bought at older rates still work at full current value. A Forever stamp purchased at 55¢ is worth $0.78 today, so you don’t add postage to compensate for the rate change itself, only for what exceeds the one-ounce letter rate.
How to Apply the Stamps
Weigh the piece on a kitchen scale, look up the rate on the USPS price calculator or Notice 123, and add stamps until you meet or slightly exceed that total. Overpaying by a few cents doesn’t get you a refund, but the mail moves without delay. For anything unusual, a postal clerk will weigh it and price it at the counter, and clerks catch nonmachinable features you might miss.
Placement matters. All stamps go in the upper right corner of the address side. A stamp partly covered by another overlapping stamp may not count toward your postage total, and USPS can treat the piece as underpaid.8USPS. DMM 604 Postage Payment Methods – USPS – Section: 1.7 Position of Stamp on Mailpiece Arrange multiple stamps side by side or in a grid so every face is fully visible.
Post offices carry the full range of denominations. The USPS online store ships individual denominations to your home. Grocery stores and pharmacies usually stock Forever stamps but rarely the smaller values you need for making up exact amounts.
If You Underpay
For domestic mail, USPS usually delivers the piece and collects the shortage from the recipient in cash as “postage due” before handing it over.9Postal Explorer. 604 Postage Payment Methods and Refunds If you included a return address, it may come back to you instead. Either way, expect a delay.
International mail is stricter. A First-Class Mail International piece underpaid by more than $1.00 with no return address goes to the Mail Recovery Center and may never reach anyone.10Postal Explorer. 423 Shortpaid Mail Always include a return address on international mail, and verify the postage before dropping it in a collection box.