In your store's Admin area, go to the Extensions > Shippings > USPS
If you offer USPS as a shipping option, your customers will see available USPS shipping rates at checkout, for delivery to their selected address.
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Display Delivery Weight: - Do you want to display the shipping weight? (e.g. Delivery Weight : 2.7674 Kg's)
Weight Class: - Set to pounds.
Length Class: - Default length class of your store. Inches or centimeters
Tax Class: - If you are required to charge tax on this item, select tax class
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