
You can even automate the tracking process for the USPS orders by sending out live tracking emails to your customers.

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. Express Mail offers only two sizes of envelope. One of the other option is to use an app if you are open to using one. The weight limit for Express Mail flat rate boxes is also 70 pounds, but the size and shape options are much more limited compared to Priority Mail. 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.
