When using Stamps.com, you'll need to select a mailpiece type to correctly calculate the cost of postage. This article provides details about the different USPS mailpiece types available so you can be sure you are making the correct choice.
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Letters: Can be up to 6-1/8" tall x 11-1/2" long, 1/4" thick, and cannot weigh more than 3.5 ounces to qualify for the First-Class Mail letter rate. If you want to send a letter using Priority Mail or Priority Mail Express, we suggest you use a Flat Rate Envelope (see below).
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Large Envelopes & Flats: Can be up to 12" tall x 15" long, 3/4" thick, cannot weigh more than 13 ounces, and must be in uniformly shaped flexible packaging to qualify for the First-Class Mail letter rate. If you want to send a large envelope or flat using Priority Mail or Priority Mail Express, we suggest you use a Flat Rate Envelope (see below).
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Flat-Rate Envelopes: These free USPS envelopes allow you to send any item, up to 70 pounds, that will fit comfortably inside. You can ship either domestically or internationally via Priority Mail or Priority Mail Express for one low rate.
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Flat-Rate Priority Mail Boxes: These free USPS boxes allow you to send any item, up to 70 pounds, that will fit comfortably inside. You can ship either domestically or internationally via Priority Mail for one low rate.
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Small: Box measures 5-3/8"x 8-5/8"x1-5/8"
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Medium: Available in either 8-1/2" x 11" x 5-1/2" or 3-3/8" x 11-7/8" x 13-5/8" sizes
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Large: Box measures 12" x 12" x 5-1/2" and includes an APO/FPO option -- ship to military posts or US Navy ships across the world for a low postage rate!
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Large Packages: The distance of the longest side plus twice the width plus twice the height is between 85" and 108". This choice alerts the Stamps.com software that your package may trigger the USPS balloon rate.
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Oversized Packages: The distance of the longest side plus twice the width plus twice the height is between 109" and 130" (USPS will not accept packages greater than 130"). These packages can only be mailed using USPS Ground Advantage (formerly Parcel Select). They are shipped under a special oversized rate that can cost as much as three times the postage for a 70-pound package.
To send a quick message to your clients or mail a reminder about upcoming appointments, a postcard is the most cost-effective method. Postcards can be up to 4-1/4" tall x 6" long and cannot weigh more than 1 ounce to qualify for the First-Class Mail postcard rate.
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NetStamps: The easiest way to print postage for postcards is to print NetStamps. You can print them in advance and use them as needed, like regular postage stamps.
To send a postcard to an international address, print NetStamps and be sure to select I want to specify the value of each stamp and choose the international option.
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Print Postage + Address: If you want to print postage and addresses at the same time, you can print directly on the postcard as if you were printing an envelope, or you can use a mailing label.
To qualify for the postcard rate, a mailpiece must:
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Be card stock only (no tri-folds, no envelopes).
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Be rectangular, satisfying the following dimensions:
Dimension
Minimum
Maximum
Height
3-1/2"
4-1/4""
Length
5"
6"
Thickness
0.007"
0.016"
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Have an aspect ratio (length divided by height) at least 1.3 but not more than 2.5.
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Be flexible and bend easily.
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Weigh no more than 1 ounce.
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Not have any items (magnets, stickers, buttons, other giveaways) that could cause a change in uniform thickness or come loose while processing.
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Have the delivery address located underneath the postage, on the same side as the postage. Any writing, decorations, endorsements, or messages should not bleed over into the space set aside for addresses and postage.
Anything that does not satisfy these requirements will be considered a letter, and you should select this as your mailpiece when using Stamps.com.
Packages: The distance of the longest side plus twice the width plus twice the height can be no more than 84".
Thick Envelopes: Identical in every way to packages, except that selecting this means your mailpiece is smaller than 1 cubic foot and therefore, you will not be prompted to enter package dimensions.
Packages can weigh as much as 70 pounds. Size and weight limits for international shipments will vary by country and mail class. For US destinations, the size requirements differ based on the package type that you select:
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Thick Envelopes: Includes bubble mailers and other large, padded envelopes and has the same size restrictions as a flat. Since a thick envelope is under 1 cubic foot, you are not required to enter package dimensions.
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Packages: Includes anything up to 84" when measuring the distance of the longest side, plus twice the width and twice the height. You must enter package dimensions for Dimensional Weight Pricing.
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Large Packages: Includes anything from 84" - 108" when measuring the distance of the longest side, plus twice the width and twice the height. You will be prompted to enter package dimensions for the Dimensional Weight Pricing.
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Oversize Packages: Includes anything greater than 108" when measuring the distance of the longest side, plus twice the width and twice the height. By selecting this option, your shipment will be charged the USPS Oversized Package Surcharge and you must select Ground Advantage (formerly known as Parcel Select) as the service.
See our USPS Content Restrictions article to determine what you can and cannot ship.
The words "large envelopes" and "flats" are used interchangeably at USPS when discussing mail that is slightly larger than regular, letter-sized mail. All large envelopes/flats must be rectangular or square-shaped and somewhat flexible. You can find out how USPS determines flexibility by referring to the USPS guide.
To mail a large envelope or flat, you must either print NetStamps or a shipping label.
Save with USPS Flat Rate Envelopes
It can be more cost-effective to use USPS Flat Rate Envelopes for heavier items. USPS Flat Rate Envelopes come in a variety of sizes, both plain and padded, as well as legal paper sizes.
For your mail to be considered a large envelope/flat and not a letter, it must exceed one or more of the letter-size minimum dimensions listed in the table below:
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Dimension |
Minimum |
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Height |
6-1/8" |
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Length |
11-1/2" |
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Thickness |
1/4" |
For your mail to be considered a large envelope/flat and not a package (parcel), it must not exceed the following criteria:
Flats Treated as Packages
Large envelopes/flats that meet the following criteria will be treated as packages and will use the Ground Advantage rate if you select the First Class Mail service:
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More than 15" long, 12" high, or 3/4" thick
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Are rigid (can't bend)
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Not uniformly thick (lumpy)
The maximum weight for large envelopes is 13 ounces. Large envelopes and flats can be shipped via Ground Advantage or Priority Mail. However, a large envelope/flat exceeding 13 ounces is classified as a Priority Mail item.
More questions about USPS letters and flats? Check out the USPS Domestic Mail Manual.
The USPS charges extra for oversized packages. If you ship a package and the Length (longest side) + 2 * Width + 2 * Height is greater than 108", select Oversized Package in the Mailpiece drop-down menu to ensure correct postage is calculated.
Depending on the weight of your shipment, the cost of shipping an oversized package can rise dramatically. You may find it more economical to break down large packages into several smaller packages if possible.
Odd-shaped packages can be difficult to measure for length, width, and height. Use the following formula: Total Size = length + girth, where length is the longest side and girth is the distance around the thickest part of the package (perpendicular to length). The examples below are a good reference.
Items that are large AND have a light weight are also subject to Dimensional Weight pricing.