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. You can even automate the tracking process for the USPS orders by sending out live tracking emails to your customers.How You Can Use USPS Flat Rate Boxes and Envelopes to Save on Shipping The rates will be displayed at checkout based on this and the shipping label can easily be generated with accurate shipping cost. You can have a look at the Multi Carrier Shipping Label app that helps you add USPS carrier and choose the Large Flat Rate Box to handle the shipping process. One of the other option is to use an app if you are open to using one. But yes, there will be issues with accuracy in the shipping cost since it is a UPS-calculated box price. One of the work around for this issue is to add a custom box with the same dimensions as of the Large Flat Rate Box and then set it as the default box so that, this box will be used to calculate the shipping cost. Have you tried contacting Shopify support regarding this issue?

Some of the merchants are able to get the Large Flat Rate Box option and some of them are not. There has been a few queries related to this issue.

Large flat rate box dimensions usps