Thanks for your attention.
Please read my previous posts - I am a experienced shipper. Please read my posts carefully, I didn't write them out of ignorance.
You are confusing Letter rate and Package rate. First Class Package rate applies to 1-15 oz (actually 16, but let's skip that), not 3.5oz.
To show this, go to USPS web site, do Mail+Ship/Calculate Price. Do United States, put in two different zip codes, then click "Calculate price based on Shape and Size". Enter 0 pounds, 6 ounces, and click PACKAGE (not letter). You'll get many options, including this one, which is what AbanteCart should be supporting. It's called First-Class Mail Parcel, at least nowadays.
It's surprising to me why this is overlooked, as most small businesses ship this way, although First-Class may be a second to Priority Mail Flat Rate or Regional Rate A.
I couldn't attach anything (?), why does the forum allow me to try? How did you do it (guess you are a moderator). View it at chickensys DOT com SLASH bin SLASH firstclassparcel.png