thats a bit like asking how to cook a steak. The answer will change depending on the cut and your preference for temperature.
the point is, if you use the first class shipping type, it is only valid for up to 13 oz. over 13 oz.s it will fail.
if you are going to use First Class, you will probably need to have a package for each item ordered.
otherwise, if you want to have one package for the entire order, you will need to use one of the other shipping types. see the USPS site for details on the shipping types and their limitations.