USPS Services and Packaging Requirements

USPS Services and Packaging Requirements

Feb 25th 2019

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

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.


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:

  • Rectangular
  • At least 3-1/2 inches x 5 inches x 0.007 inch thick
  • No more than 6-1/8 inches high x 11-1/2 inches long x 1/4 inch thick

2. Flats or Large Envelopes

Flats are mailings that are:

  • Rectangular
  • Have one dimension that is greater than 6-1/8 inches high OR 11-½ inches long OR ¼ inch thick.
  • Be no more than 12 inches high x 15 inches long x ¾ inch thick.

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).
  • Maximum weight is 70 pounds.

USPS provides a handy chart summarizing the above, copied here and available at this link:

USPS Mailer Chart


USPS provides an extensive set of basic standards for all mailings, which can be found here:

A summary of 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-machineable and subject to additional charges.

Flats or large envelopes: Paper envelopes, plastic bags / poly mailers, paper wrap, corrugated boxes, or cloth can be used for these mailings. Poly mailers must be at least 2 mil thick for shipments up to 5 lbs and 4 mil 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 laying flat. Any paper used for paper bag or paper wrap packaging must be 50#.

Parcels: Corrugated 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 mil thick for shipments up to 5 lbs and 4 mil 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#.

When the above standards aren't met, if maximum weights a 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.

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 a package that can qualify as a "letter", the package must be 3.5 oz or less. If you are shipping something that qualifies as a 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. 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. The shipping cost is based on distance, and is calculated by the number of zones a mailing has to cross in order 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 provides the following handy chart summarizing shipping methods at this link:

Choosing a Service for Mailing

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


Last month, a variety of price changes went into effect for USPS, some of which are quite steep. A great summary of these pricing changes can be found here: 

  • The First Class Mail letter (1 oz.) rate for postage purchased at the Post Office is increasing by five cents to $0.55 from $0.50.Each additional ounce for a First Class Mail letter will cost an extra $0.15 (a decrease from $0.21 in 2018).
  • First Class Mail Flats/Large Envelope rates are not increasing in 2019, remaining at $1.00 (1 oz.), with additional ounces costing $0.15.
  • Priority Mail Express will see an average increase of 3.9%. A 1 lb. package going to Zone 8 using Priority Mail Express will cost $36.52 in 2019 compared to $35.04 in 2018. USPS is also adjusting its dimension weight divisor from from 194 to 166, so large lightweight packages will be penalized more than they have been in the past. Commercial Plus Pricing has been eliminated.
  • Priority Mail will see an average increase of 5.9%. For example, a 1 lb. package going to Zone 8 using Priority Mail will cost $8.25 in 2019 compared to $7.76 in 2018. The rate increase is most prevalent for lightweight shipments (up to 4 lbs). Parcels and large envelopes over 4 lbs may actually see a decrease. However, in June 2019, USPS will reduce the dimensions weight divisor down from 194 to 166.