The number of Forever stamps for a bubble mailer depends on its finished mail class, not just its 6×9 size or weight. A qualifying 3-ounce flat needs three whole Forever stamps at the current rate. At the July 12, 2026 rates, a 3-ounce flat costs $2.27; three 82-cent Forever stamps supply $2.46 and overpay by 19 cents. A rigid, lumpy or nonuniform bubble mailer is a parcel and uses zone-based USPS Ground Advantage pricing instead.
First classify the finished mailer
| Finished mailpiece | Likely class | How to calculate |
|---|---|---|
| Flexible, rectangular, uniformly thick; over 1/4 inch but no more than 3/4 inch; within flat dimensions | Large envelope (flat) | Use First-Class Mail flat price by ounce |
| Rigid, lumpy, not uniformly thick, too thick, or not flexible enough | Parcel | Use Ground Advantage price by weight and zone |
| Letter-size and no more than 1/4 inch thick but rigid or uneven | May be nonmachinable or a parcel | Have USPS inspect the sealed piece |
3-ounce flat calculation
The current 3-ounce large-envelope price is $2.27. Three current Forever stamps equal $2.46. To avoid overpaying, use two Forever stamps ($1.64) plus 63 cents in other valid postage. Rates and Forever values can change, so recalculate on the mailing date.
What about a 6×9 bubble mailer?
Size alone does not settle the class. A 6×9 piece can be letter-size, but bubble padding and contents may make it thicker than 1/4 inch, rigid or uneven. If it does not satisfy the applicable letter or flat standards, USPS charges parcel pricing. Weigh and measure the sealed mailer with the contents already inside.
For a sealed 6×9 mailer weighing 3 ounces, these are different outcomes at the July 12, 2026 rates:
| Verified mail class | Postage | Whole Forever stamps |
|---|---|---|
| Qualifying machinable letter, no thicker than 1/4 inch | $1.40 | 2 stamps pay $1.64; 24 cents extra |
| Qualifying flexible, uniformly thick flat; over 1/4 inch and no thicker than 3/4 inch | $2.27 | 3 stamps pay $2.46; 19 cents extra |
| Parcel, or classification not established | Obtain a service and ZIP-specific quote | No fixed count from 6×9 or 3 ounces alone |
The letter example excludes nonmachinable surcharges. Do not select a lower class merely because its postage is cheaper.
If it is a parcel
Do not use the flat calculation. Retail Ground Advantage begins at a 4-ounce price tier and varies by destination zone. Use a shipping label or present the sealed parcel at a Post Office; guessing with a fixed number of Forever stamps can underpay the piece.
If a package uses postage stamps and is over 10 ounces or thicker than one-half inch, hand it to a Post Office retail employee. See the USPS stamped-package drop-off instructions; sufficient postage does not remove this requirement.
Four checks before mailing
- Weigh the sealed mailer, rounding up to the next applicable ounce or price tier.
- Measure length, height and maximum thickness.
- Check flexibility and whether contents create more than 1/4-inch thickness variation.
- Enter the destination and mailpiece type in the USPS calculator or request counter verification.
For other shapes and destinations, use our Forever stamp calculator. See also the broader guide to using Forever stamps on packages.
Official sources
The current USPS Notice 123 price list gives First-Class Mail flat and Ground Advantage retail prices. DMM 101 defines letter, flat and parcel physical standards, and DMM 133 explains that flats failing those standards pay parcel prices.
Rates checked September 9, 2026; current Notice 123 effective July 12, 2026.