Innovators in Eco-Friendly Packaging and Shipping Supplies

USPS Services and Packaging Requirements in 2021

USPS Services and Packaging Requirements in 2021

Dec 20th 2021

Businesses typically have three shipping partners to choose from - USPS, UPS, and FedEx. For many (especially those who ship a lot of lightweights similarly sized and shaped items), USPS is the cheapest option.

EcoEnclose is proud to work with USPS on many of our shipments. We have found that their pricing is hard to beat for our sample and smaller orders, and that they are great to work with. In our work with them, we’ve learned a lot about how to package things up – what mailer size to use, which ones to avoid.

As with any shipper, USPS has a myriad of different service options and packaging requirements, some of which are confusing to understand unless you have hours to dig into their website.

Here, we break down some of the most important things to know when it comes to packaging for USPS, based on our research and our own experiences.

Specifically, we cover:

  • Categories of shipments – what qualifies as a letter versus a flat versus a parcel
  • Minimum packaging standards for each shipment category
  • Shipping services that USPS offers


USPS has three main categories of mailings, defined by the shape, size, and weight of a shipment.

1. Letter

Letters are any mailing that is:

  • At least 3-1/2 inches x 5 inches x 0.007 inches thick
  • No more than 6-1/8 inches high x 11-1/2 inches long x 1/4 inch thick
  • Standard letters are rectangular, with an aspect ratio 1.3 and 2.5. While mailings that fall out of these aspect ratios (including square envelopes) count as “letters”, they are subject to non-machinable surcharges.

2. Flats or Large Envelopes

Flats are mailings that are:

  • Have one dimension that is greater than 6-1/8 inches high OR 11-½ inches long OR ¼ inch thick.
  • Are no more than 12 inches high x 15 inches long x ¾ inch thick.
  • While USPS dictates that flats or large envelopes must be rectangular, square mailings are accepted as “rectangular” in this instance. This shipment category is not subject to aspect ratio requirements.

It is helpful to note the 12x15” maximum sizing. EcoEnclose offers a number of mailers at or under this size, including our Apparel Mailer (7" x 9", 10" x 13", 12" x 15"), Kraft Mailer (6 x 10", 8.5 x 12"9.5 x 14.5"), EcoX Mailer (6 x 9", 7.5 x 10", 9 x 12", 10 x 13", 12 x 15"), and Rigid Mailers (6" x 8", 9" x 11.5", 11" x 13.5").

3. Parcels

Parcels are mailings that are:

  • At least 3 inches high x 6 inches long x 1/4 inch thick.
  • Mailing tubes can be used as long as the length is no more than 10 times the width of the tube.
  • No more than 108 inches in length and girth combined (130 combined for USPS Retail Ground). Length is the measurement of the longest dimension and girth is the distance around the thickest part (perpendicular to the length).
  • The maximum weight is 70 pounds.

USPS provides this handy chart summarizing the above.

USPS Mailer Chart


USPS provides an extensive set of basic standards for all mailings.

A summary of what we believe are the most relevant acceptable and machinable package types are as follows:

Letters: Paper. Any non-paper envelope (such as a poly mailer) that is shipped as a "letter" would be considered non-machinable and subject to additional charges.

Flats or large envelopes: Paper envelopes, plastic bags/poly mailerspaper wrapcorrugated boxes, or cloth can be used for these mailings. Poly mailers, like our 100% Recycled Ivory Poly Mailers, must be at least 2 mils thick for shipments up to 5 lbs and 4 mils thick for shipments up to 10 lbs (and are not acceptable for shipments over 10 lbs). When poly mailers are used, their overhang should not exceed 0.5 inches when the package is lying flat. Any paper used for paper bag or paper wrap packaging must be 50# in strength.

Parcels: Corrugated shipping boxes are the most common packaging option for parcels. For parcels that are over 3/4 in thick (or exceed other dimensions required to qualify as a large envelope/flat), but are still relatively thin could be packaged in expandable paper envelopes, poly mailers / plastic bags, paper bags or wrap, or even cloth. Poly mailers must be at least 2 mils thick for shipments up to 5 lbs and 4 mils thick for shipments up to 10 lbs (and are not acceptable for shipments over 10 lbs). Any paper used for paper bag or paper wrap packaging must be 50# in strength.

When the above standards aren't met, if maximum weights and dimensions are exceeded, or if a package is oddly shaped or irregular, it will likely incur a "non-machinable" and/or "irregular parcel" surcharge.


USPS has four main categories of shipping services that are relevant for an eCommerce business. The actual price of shipping something will vary based on the service selected, the mailing type (letter, large envelope, or parcel), the actual weight and dimensions of the mailing, and the distance it will travel. However, the following section clarifies key points.

First Class: This service can be used to ship letters, large envelopes/flats, or parcels. Typically, this is the most inexpensive way to ship something and the best way to ship something light. If you are shipping something First Class that can qualify as a "letter" shipping category, the package must be 3.5 oz or less. If you are shipping something that qualifies as a flat, large envelope or a parcel, it must be 13 oz or less. First-class pricing is not dependent on how far mail travels, which makes it a great fit for companies that offer free shipping on lightweight items because you know ahead of time what your shipping costs on these items will be. First-Class Mail postage includes forwarding and return services. You can also add extra services such as Registered Mail and Certified Mail.

Priority: This service can be used to ship letters, large envelopes/flats, or parcels. Priority Mail can be used for any shipment under 70 lbs, and it must be used for any mailing over 13 oz (or a letter over 3.5 oz), which are too heavy for First Class Mail. In addition to the weight flexibility, priority mail has the added benefit of being delivered within 1-3 business days anywhere in the United States. The shipping cost is based on the distance, and is calculated by the number of zones a mailing has to cross to reach its final destination. Priority Mail also includes USPS tracking free of charge. It includes forwarding and return service, and you can add registered mail.

Priority Express: This service can be used to ship letters, large envelopes/flats, or parcels, and is valid for any shipment under 70 lbs. Priority Mail Express is the fastest mail service offered by the Postal Service. It provides guaranteed 1-Day or 2-Day expedited service by 3 p.m. for any mailable matter and includes $100 of insurance coverage. Priority Mail Express delivery is offered 365 days a year in many locations.

Parcel Select: Parcel Select is only available for parcels, and only for shipments that are booked online (i.e. for companies that have an online account set up) or when a company has a minimum of 50 pre-sorted shipments teed up in one transaction. While it is relatively inexpensive, it is also relatively slow, with a 2-8 day delivery window. There is no forwarding or return service, so if a recipient has moved, the package is unlikely to find its way back to the sender or the buyer. 

Media Mail: Media Mail is USPS's cost-effective way to send packages that contain media and educational materials (which are typically quite dense and heavy). It is available for parcels weighing up to 70 lbs. Only the following items can be shipped via Media Mail: 

  • Books (at least eight pages)
  • Printed music and test materials
  • Video and sound recordings
  • Printed educational charts
  • Medical loose-leaf pages and binders
  • Computer-readable media 

By shipping via Media Mail, you are consenting to have your package contents inspected and verified.

USPS Retail Ground: Historically referred to as Parcel Post or Parcel Select, this shipping class is for small and large packages, thick envelopes, and tubes They can be shipped domestically using USPS Retail Ground.

USPS provides this handy chart summarizing shipping methods.

Choosing a Service for Mailing

To learn more about USPS's shipping services for Small Businesses, check out their Business Mail 101 Portal.


In August 2021, USPS announced a myriad of pricing changes. Below is a recap of these changes. Learn more here.

First Class Mail Pricing

USPS Package Services

USPS Special Services


Shop all of our recycled mailers here. Here are a few mailer options to get started.

100% Recycled Padded Mailer - 10.5 x 15"

  • Total Package Weight - 13 oz or more
  • Shipping Service - Priority
  • Package Type - Flat/Envelope

100% Recycled Paper Apparel Mailer - 12" x 15"

  • Total Package Weight - 13 oz or more
  • Shipping Service - Priority
  • Package Type - Flat/Envelope

100% Recycled Kraft Mailer - 6 x 10"

  • Total Package Weight - 3.5 oz or less
  • Shipping Service - First Class
  • Package Type - Letter

Feeling overwhelmed? Contact us today. We are happy to help!