USPS Priority Mail Flat Rate Envelope Rules(A Must-Read)

We all need to mail one thing or another from time to time. Mail service has become quite advanced in recent years introducing a variety of different packaging options, including the flat rate envelope one.

In order to utilize the USPS Priority Mail Flat Rate service, it is important to follow their rules. The basic rule is that whatever you are shipping must fit into the given envelopes, without those envelopes being modified in any way. Tape cannot be used to hold the envelope shut. It should not be overly bulky. It must be under 70 lb.

We will tell you all about this type of mailing and answer a few of the most common questions.

What Are The Priority Mail Flat Envelope Rules?

There are a few rules for using these envelopes, as follow:

  • You should be able to easily close the provided envelope with the items that you have put inside it.
  • Tape cannot be used to hold the envelope shut.
  • It should not be overly bulky.
  • It must be under 70 lb.

To explain in more detail, the first point means that the packet must be sealable without stressing the envelope. If you are having to crush the items in and squeeze the end over, it is probably too tight for those items.

If this is the case, you should not ship them using a Flat Rate envelope.

The second bullet point is an extension of the first. If your envelope will not stay shut without using tape to force it shut, your items are definitely too big for a Flat Rate envelope.

The third bullet point is trickier to explain because USPS has not given any strict guideline on how fat a Flat Rate envelope is allowed to be. The company has simply said that packets should not be stuffed too full.

Finally, packages must not exceed 70 lb. This is true irrespective of which kind of envelope you use; 70 lb. is the maximum weight for a Flat Rate envelope.

What Is Allowed In A Flat Rate Envelope?

These envelopes are commonly used for shipping documents, but you can use them for shipping anything legal, provided you do not over-fill them or exceed 70 lb.

There are no stipulations that say you cannot send household items and gifts that fit in these envelopes.

This can make shipping much cheaper for some businesses and commercial shippers, and also for everyday people.

How Do I Ship A Priority Mail Flat Rate Envelope?

The first thing you need to do is source the envelope. You can get them for free from USPS offices, or from the USPS website in packs of 5, 10, or 15.

To mail the envelope, you should pack your items in it, and then you have two options. You can:

  • Purchase pre-paid labels for the package online, print them out, and stick them to the parcel.
  • Go to a post office with the parcel and get the shipping clerk to help you purchase the appropriate mailing label for it.

If you are printing the label at home, be aware that you will need to tape the labels on unless you have a sticky label machine. You can request for your package to be picked up from your home address if this is easier than going to a post office.

Related Post: USPS Priority Mail Flat Rate Box Rules(All You Need To Know)

Q&A Section:

Here are a few of the commonly asked questions about Priority Mail Flat Rate shipping.

Can Flat Rate Envelopes Bulge?

Yes, a Flat Rate envelope can bulge. The word “flat” refers to the fact that you will be charged a flat (fixed) fee, regardless of where you are mailing it to in the US. It does not refer to the shape that the package must be.

USPS used to use the slogan, “if it fits, it ships.” This implied that any amount of bulge was acceptable. However, in 2016, the company issued a warning telling people not to over-stuff the envelopes.

At that time, they stated that a small bulge was acceptable, but very full or modified envelopes would not be sent at the flat rate.

Can I Use Duct Tape On A Flat Rate Envelope?

No, you should not use duct tape on any packages that you intend to send with USPS or any other mail carrier. Duct tape is not considered secure enough for sending packages with, and USPS will reject any mail that is packaged using duct tape. This is true of Flat Rate envelopes and other mail types.

You can use normal tape on a Flat Rate envelope, but only for reinforcement. You may tape up the seams and the flap, but you cannot modify the envelope using tape, or it will be sent as zoned Priority Mail.

Related Post: Can You Use Duct Tape On Packages?(Not Really)

Can You Use Any Envelope For Priority Mail?

If you choose to, you can use your own envelope for Priority Mail. However, you will need to add the “Priority Mail” marking, using bold, legible writing or a sticker.

Note that if you want to use Priority Mail Flat Rate, you must use an envelope provided by USPS. This is because there are stipulations about the envelope size, and USPS needs to know that these are being met without having to measure every envelope. Therefore, they only allow customers to access this service when they use branded envelopes.

Is It Cheaper To Use Your Own Envelope For Priority Mail?

In some circumstances, it is cheaper. This usually depends on which Zone you are shipping to. Some of the low number zones will be cheaper if you use your own envelope, rather than using a Flat Rate one. It is best to check this with the shipping clerk.

How Many Stamps Do I Need For A Priority Mail Flat Rate Envelope?

You will need to put stamps totaling the current price of a Priority Mail Flat Rate Envelope. In 2021, this was $7.95, so make sure that you add enough stamps. Always check current postal rates, as these are subject to change.

Can I Use A Priority Mail Envelope For First Class?

No. Priority Mail packaging is only to be used for the Priority Mail service. USPS provides this packaging for free, and it must not be used for any other method of shipping.


The USPS Priority Mail Flat Rate envelope rules are relatively straightforward. If the item fits comfortably in the package provided by USPS, it can be mailed in it, provided it weighs less than 70 lb.