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Carriers & Rates
USPS Flat Rate Boxes, Envelopes, and Prices
USPS Flat Rate boxes and envelopes allow you to ship up to 70 lbs with USPS Priority Mail® at one low rate to anywhere in the US. The advantage of Flat Rate is that regardless of how much you fit into the packaging, it’s allowable. There are times when USPS Flat Rate boxes are the cheapest way to ship. But, there are also times when standard Priority Mail, or other services, works better. Regardless, knowing how to use Flat Rate shipping options will end up saving you time and money on your shipping.
What Is USPS Flat Rate?
So, how do USPS Flat Rate boxes work? USPS Flat Rate is a pricing model under Priority Mail® where you pay one flat fee to ship packages or envelopes of specific dimensions, regardless of weight (up to 70 lbs) or destination within the U.S.
These USPS Priority Mail Flat Rate services include:
- Flat Rate Envelopes (standard, legal, padded).
- Flat Rate Boxes (small, medium, large).
- APO/FPO options for military addresses.
Because rates are fixed, USPS Flat Rate shipping is ideal for heavy shipments going long distances.
Where to Get USPS Flat Rate Boxes
USPS Flat Rate boxes and envelopes are free—you can order them online from the USPS store or pick them up at most Post Office locations. Just make sure you use the packaging with the corresponding Priority Mail Flat Rate service. Misusing flat rate packaging (e.g., sending it via First-Class or Media Mail) can result in returned parcels or billing corrections.
When Can I Not Use USPS Priority Mail Flat Rate Shipping Boxes?
USPS Flat Rate packaging CANNOT be used with standard Priority Mail services, First Class Mail®, Media Mail®, or with other carriers. They are only allowed for their designated Priority Mail Flat Rate service. If you try to use a Flat Rate box or envelope with a standard Priority service or any other service, the parcel will be returned or a post-billing correction will be sent to your postage account.
What are the USPS Flat Rate Box Prices?
Priority Flat Rate is the USPS’s one-size-fits-all packaging that lets you fit as much as you can into the box as long as it’s under 70 lbs. The prices for Priority Mail Flat Rate and dimensions of each package type are listed below.
USPS Flat Rate Box Sizes & Envelope Options
Here’s a breakdown of USPS Flat Rate box sizes, flat rate envelope types, and current pricing:
| USPS Priority Flat Rate Type | ShipStation/Commercial Rate | Outer Dimensions* | Retail Rate** |
|---|---|---|---|
| Flat Rate Envelope | $8.05 | 12 1/2″ x 9 1/2″ | $9.65 |
| Window Flat Rate Envelope | $8.05 | 10″ x 5″ or 12 1/2″ x 9 1/2″ | $9.65 |
| Small Flat Rate Envelope | $8.05 | 10” x 6” | $9.65 |
| Legal Flat Rate Envelope | $8.30 | 15″ x 9 1/2″ | $9.95 |
| Padded Flat Rate Envelope | $8.80 | 12 1/2″ x 9 1/2″ | $10.40 |
| Small Flat Rate Box | $8.55 | 8 11/16″ x 5 7/16″ x 1 3/4″ | $10.20 |
| Medium Flat Rate Box (top-loading) | $14.75 | 11 1/4″ x 8 3/4″ x 6″ | $17.10 |
| Medium Flat Rate Box (side-loading | $14.75 | 4″ x 12″ x 3 1/2″ | $17.10 |
| Large Flat Rate Box | $19.90 | 12 1/4″ x 12 1/4″ x 6″ | $22.80 |
| APO/FPO/DPO Large FRB | $18.35 | 12 1/4″ x 12 1/4″ x 6″ | $21.20 |
**The rates available through the Post Office, ShipStation provides discounts on select USPS services
How Much Do Priority Mail Express Flat Rate Envelopes Cost?
Currently, the USPS offers a few flat rate options for its USPS Priority Mail Express service. This is a quicker alternative to their Priority Flat Rate offerings and is typically delivered overnight. With a maximum weight of 70 lbs, you can still use the USPS Flat Rate envelope, Legal Flat Rate envelope, or Padded Flat Rate envelope. However, like Regional Rate boxes, the USPS has discontinued its Priority Mail Express Flat Rate Box offerings.
USPS Priority Mail Express Flat Rate Envelopes
For urgent shipments, USPS offers Priority Mail Express Flat Rate envelopes. These provide overnight to 2-day delivery, 70 lb weight limits, and include insurance and tracking.
| USPS Flat Rate Type | Flat Rate Dimensions | Flat Rate Pricing | Flat Rate Max Weight |
|---|---|---|---|
| Flat Rate Envelope | 12 ½″ × 9 ½″ | $24.90 | 70 lbs |
| Legal Flat Rate Envelope | 12 ½″ × 9 ½″ | $25.15 | 70 lbs |
| Padded Flat Rate Envelope | 12 ½″ × 9 ½″ | $25.35 | 70 lbs |
| Flat Rate Box Top Loading | 11 ¼″ × 8 ¾″ × 6″ | discontinued | 70 lbs |
| Flat Rate Box Side Loading | 12″ × 3 ½″ × 14″ | discontinued | 70 lbs |
USPS Regional Rate Boxes (Discontinued)
As of January 22, 2023, USPS Regional Rate Boxes A and B have been discontinued. These previously provided zone-based pricing for smaller, heavier packages. Merchants now turn to flat rate boxes USPS or discounted USPS Priority Mail through platforms like ShipStation.
Flat Rate vs. Standard Priority Mail: When to Choose What
USPS Priority Mail is the standard option for the USPS to send parcels under 70 lbs and do not exceed 108″ in combined length and girth (girth = the distance around its thickest part). These parcels will reach their final delivery in the same timeframe as the flat rate options. However, their prices are structured differently. Whereas flat rate boxes don’t have weight, dimensions, or delivery zone factored into them, standard Priority Mail parcels do. For lighter parcels, it’s usually less expensive to use your own packaging and get a rate.
| Factor | USPS Flat Rate | Standard Priority Mail |
|---|---|---|
| Weight Sensitivity | Great for heavy items | Ideal for lightweight items |
| Box Size | Predefined dimensions only | Flexible—you provide the box |
| Cost Calculation | Flat rate (no zone/distance pricing) | Based on weight, zone, dimensions |
| Speed | 1–3 business days | 1–3 business days |
For example: a 3 lb package from NY to CA might cost less with a user-supplied box vs. USPS medium flat rate box price.
USPS Flat Rate International Shipping
USPS also offers Flat Rate International options through Priority Mail International and Priority Mail Express International. These let you send packages to over 180 countries with tracking and up to $200 insurance.
| International Flat Rate | Service | Approx. Delivery | Sample Price (Retail) |
|---|---|---|---|
| Flat Rate Envelope | Priority Mail Intl | 6–10 business days | ~$31.80 |
| Flat Rate Box (Medium) | Priority Mail Intl | 6–10 business days | ~$79.60 |
| Flat Rate Box (Large) | Priority Mail Intl | 6–10 business days | ~$100.65 |
| Flat Rate Envelope | Priority Mail Express Intl | 3–5 business days | ~$54.75 |
| Flat Rate Box (Medium) | Priority Mail Express Intl | 3–5 business days | ~$91.50 |
| Flat Rate Box (Large) | Priority Mail Express Intl | 3–5 business days | ~$113.25 |
Tip: Use ShipStation’s multi-carrier tools to automate customs documentation and compare international flat rate options from USPS and other carriers.
USPS Flat Rate Boxes – FAQs
How much is a flat rate box at USPS?
USPS flat rate box prices vary by size. As of 2025 retail pricing:
- Small Flat Rate Box: $10.20
- Medium Flat Rate Box: $17.10
- Large Flat Rate Box: $22.80
How do I purchase a pre-paid USPS flat rate envelope?
To buy a prepaid USPS flat rate envelope, you can:
- Visit a USPS location and request a pre-paid envelope.
- Buy postage online through USPS.com or platforms like ShipStation, which lets you print prepaid labels directly and schedule pickups.
How much is a USPS flat rate envelope?
A standard flat rate envelope is $9.65 at retail, or $8.05 with ShipStation’s discounted commercial rate.
What is USPS flat rate packaging?
USPS Flat Rate packaging includes standardized boxes and envelopes that let you ship up to 70 lbs at one flat rate. Prices are fixed regardless of weight or destination within the U.S.—just make sure to use the proper Priority Mail Flat Rate label.
Save on USPS Flat Rate Shipping with ShipStation
Using flat rate shipping boxes is already cost-effective—but ShipStation helps you save even more. Through ShipStation Carrier Services, you get access to:
- Up to 89% off USPS retail rates.
- Pre-built shipping automation rules.
- Multi-carrier rate comparisons.
- Batch label printing & scheduled pickups.
Whether you’re using USPS prepaid boxes, padded mailers, or shipping by weight, ShipStation makes it easy to ship smarter, not harder.
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