One Forever stamp covers an ordinary letter up to 1 oz. Above that you are adding 29 cent additional-ounce stamps, and a 9x12 manila envelope starts at three stamps because USPS prices flats from $1.69. As soon as the thing you are sending is rigid, lumpy or over 13 oz it leaves the letter world entirely and needs a package label, which on 2026-07-28 meant $8.48 for a boxed mug from Newark to Atlanta. That parcel side is all we do: SMKlog sells domestic package labels with no account, and sells no stamps, no mailbox service and no international shipping.
That single test decides whether stamps can pay for the piece at all.
Forever stamps are worth $0.82 each, so stacking them onto a heavier letter overpays unless the arithmetic happens to land. The cleaner method is one Forever stamp plus additional-ounce stamps, which are priced at the 29 cent step between the published weights. Both routes are in the table.
| Weight | Notice 123 price | Exact combination | Forever stamps if that is all you have | You would pay | Overpaid by |
|---|---|---|---|---|---|
| 1 oz | $0.82 | 1 Forever stamp | 1 | $0.82 | $0.00 |
| 2 oz | $1.11 | 1 Forever stamp plus 1 additional-ounce stamp | 2 | $1.64 | $0.53 |
| 3 oz | $1.40 | 1 Forever stamp plus 2 additional-ounce stamps | 2 | $1.64 | $0.24 |
| 3.5 oz | $1.69 | 1 Forever stamp plus 3 additional-ounce stamps | 3 | $2.46 | $0.77 |
A letter cannot go past 3.5 oz. USPS states the maximum weight for First-Class Mail letters is 3.5 oz, and a heavier flat piece is priced on the large envelope ladder instead. Watch for the $0.49 non-machinable surcharge as well, which lands on square envelopes, rigid ones and anything with a clasp, and which no stamp count in the table above includes.
A 9x12 or 10x13 envelope is what USPS calls a flat, and flats have their own price ladder that starts higher and climbs about 30 cents an ounce. The stamp counts get ugly quickly, because $0.82 rarely divides neatly into a flat price.
| Weight | Notice 123 price | Forever stamps needed | You would pay | Overpaid by |
|---|---|---|---|---|
| 1 oz | $1.69 | 3 | $2.46 | $0.77 |
| 2 oz | $1.98 | 3 | $2.46 | $0.48 |
| 3 oz | $2.27 | 3 | $2.46 | $0.19 |
| 4 oz | $2.56 | 4 | $3.28 | $0.72 |
| 5 oz | $2.85 | 4 | $3.28 | $0.43 |
| 6 oz | $3.14 | 4 | $3.28 | $0.14 |
| 7 oz | $3.43 | 5 | $4.10 | $0.67 |
| 8 oz | $3.72 | 5 | $4.10 | $0.38 |
| 9 oz | $4.02 | 5 | $4.10 | $0.08 |
| 10 oz | $4.32 | 6 | $4.92 | $0.60 |
| 11 oz | $4.62 | 6 | $4.92 | $0.30 |
| 12 oz | $4.93 | 7 | $5.74 | $0.81 |
| 13 oz | $5.24 | 7 | $5.74 | $0.50 |
The 12 oz row is the one to look at. Six stamps come to $4.92 against a $4.93 price, so the piece needs a seventh and you hand over $5.74 for postage worth $4.93. Anything that lands on a wasteful row is worth taking to a counter or a meter, where the exact price gets printed on it instead. To stay a flat at all, the envelope has to be uniformly thick and bend easily. Put something rigid inside and USPS prices it as a package no matter what the scale says.
USPS caps large envelopes at 13 oz, and the flat ladder above ends there for that reason. Two other things end the stamp conversation early: a rigid object inside the envelope, and the USPS rule that a mailpiece over 10 oz carrying only stamps is not eligible for free package pickup. At that point the piece needs a package label with a tracking number on it, so here is what one costs on a live lane.
| Carrier | Service | Price | Transit window returned |
|---|---|---|---|
| USPS | Ground Advantage | $8.48 | 8-9 business days |
| UPS | Ground Saver | $10.18 | Not supplied |
| UPS | Ground | $10.67 | 2-3 business days |
| FedEx | Ground Economy | $12.42 | 6-7 business days |
| UPS | 2nd Day Air | $16.58 | 2-3 business days |
Set that $8.48 against the $5.24 that 13 oz of flat mail costs and the trade is visible. The parcel label buys tracking, a wider weight range and three carriers competing for the job, and the flat rate buys none of those things but costs less while the piece stays flat. Paper stays in the mail stream, objects go on a label. For padded mailers specifically, our guide to shipping in a bubble mailer covers what fits and what the carriers do with it, and the cost of a shipping label takes the pricing question wider.
Twenty sheets of ordinary copy paper plus an envelope sits close enough to the 3.5 oz letter ceiling that guessing is a coin toss. A kitchen scale settles it in five seconds and saves the piece coming back marked for postage due.
Padding makes a mailer thick and uneven, which is exactly what the flat definition excludes. Sellers put a couple of stamps on a bubble mailer every day and it is the single most common reason a small shipment reappears on the doorstep.
Overpaying by fifty or eighty cents on one envelope is not worth a trip anywhere. Doing it across a run of forty client letters is around thirty dollars of stamps burned, and at that volume a meter or a counter visit pays for itself immediately.
One Forever stamp covers a letter up to 1 oz, which USPS prices at $0.82. Each further ounce adds $0.29, so a 2 oz letter is $1.11 and a 3.5 oz letter, the heaviest a letter can be, is $1.69.
Three at the lightest. USPS prices a 1 oz large envelope at $1.69, and three Forever stamps come to $2.46, so you overpay $0.77 unless you buy exact postage. The ladder climbs about 30 cents an ounce from there and ends at $5.24 for 13 oz.
It covers a letter that weighs between one and two ounces in a single stamp, which is why wedding and event stationery is usually mailed with one. USPS prices that 2 oz letter at $1.11 against $0.82 for a single ounce, and the same result comes from one Forever stamp plus one additional-ounce stamp.
Once the piece passes 13 oz, or as soon as anything rigid goes inside it. USPS also excludes mailpieces over 10 oz bearing only stamps from its free package pickup. On 2026-07-28 a boxed mug at 2 lb from Newark to Atlanta quoted $8.48 at the cheapest, with tracking attached.