Priority Mail does not have separate rates for letters, large envelopes and packages; that's just for First Class. So you don't have to worry about that if you're shipping via Priority Mail (i.e., if the packaged magazines weigh over 16 ounces). Just list them as "Package/thick envelope."
If the packaged items weigh under 16 ounces, they can be mailed as First Class packages. Be sure to put some cardboard in the envelopes, so they don't get bent. I recommend two pieces of corrugated cardboard, with the "grain" going crosswise to each other for extra rigidity.
