That being said, let's dive into the different USPS Priority Mail Box and Envelope options. With a USPS Flat Rate Box or Envelope, you will be able to ship any item of any weight up to 70lbs to any location for the same price assuming it fits properly, without any bulging, within its designated box or envelope. If using non-customized boxes is not a deal breaker for you, USPS Flat Rate Shipping is a great tool to spend less on shipping. This can be a drawback for some because, unless you are a high volume shipper, you cannot get customized boxes with specific branding. In order to have access to the USPS Priority Mail Flat Rate pricing, you must use one of USPS designated Flat Rate Boxes you cannot use just any box of the same dimensions. You can pick them up at any post office or USPS will actually ship the boxes opens in new tab open_in_new to you for free. One of the many benefits of shipping with USPS Flat Rate Boxes is that the boxes themselves are free. How You Can Use USPS Flat Rate Boxes and Envelopes to Save on Shipping

Priority mail flat rate envelope rules