
You can even automate the tracking process for the USPS orders by sending out live tracking emails to your customers. Ship from Post Office locations or from your home or business with Click-N-Ship service. The rates will be displayed at checkout based on this and the shipping label can easily be generated with accurate shipping cost. Priority Mail Flat Rate lets you ship packages up to 70 lbs to any state at the same price. 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. If your package would be cheaper to ship through regular Priority Mail but you use a flat rate box, you still pay the flat rate cost. 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. When a flat-size piece of mail is a box or has contents that make the mailpiece rigid, it is classified and priced as a package. When a First-Class Mail item exceeds any one of the maximum measurements of a large envelope, it is classified and priced as a package (parcel). 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. A large envelope exceeding 13 ounces is classified as a Priority Mail item.

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. New for 2021, however, a 100 fee will be assessed on parcels found in the mailstream that exceed the maximum mailable size limit (combined length and girth greater than 130 inches).
