I only use USPS, as my products are light and small. First class envelope does not work as my items usually exceed the height requirement.
So I use First class package as my shipping, and give priority and priority express as alternatives to quicker shipping.
I have all 3 rates chosen, however when I go to checkout, First class package only works on some of my items, not all of them.
I do not have special rates for each product, and its is mere ounces that differ. I cant open my shop because I would never ask someone to pay more for shipping then what I sell my products for. Not this anyways.