We obtain USPS rates by querying the USPS.gov rate API. The rates we're calculating that you listed above are accurate for the USPS First Class package retail rate, as you can see at: http://postcalc.usps.gov/



We can also support USPS Express, Priority, Parcel Post and Media Mail retail rates, and you could even choose to offer buyers a selection of multiple rates. We do not support Commercial Base rate lookups; however, there's no inherent reason you couldn't have the cart look up retail rates and actually ship at your Commercial Base rate for the same service.



To add Shipping Rules for additional methods, go to Seller Admin > Cart Shipping Settings and click Next to reach the Shipping Rules screen, where you can define a Rule for each new shipping method/rate you want to offer. If a buyer's cart order qualifies for more than one Rule, they would see a menu in their cart offering a selection of rates/methods.

If you set up rules using any of our USPS shipping rate calculator methods, you don't need to worry about setting up separate rules for every weight class, destination country, etc., as we would query USPS.gov for each order's destination, weight, and number of packages (determined by your products' selected Packaging Type and Capacity). If a given service isn't available for an order's weight (e.g., parcels over 13 oz. cannot go First Class), then we would not offer the buyer a rate for that service.



However, you can set up as many rules as you need, and if you prefer to set up custom "Price per unit weight" rates, those can run to 11 characters long (so up to 10 decimal places assuming a leading decimal point). USPS and "Price per unit weight" rate calculations do factor in the Cost and Weight of any packaging type(s) selected for items in the order. See our Shipping help page for a fuller explanation of how different products can consolidate into common packaging types together:

http://www.e-junkie.com/ej/help.shipping.htm