My local post office will not mail any bubble packs or cardboard photo mailers via regular mail, they ALWAYS charge parcel rates either domestic or foreign per the new rates. I've talked to the post master and even the regional distribution center and got the same answer.

One solution I've been using for slot cards is a standard 6x9 manila envelope with some cardboard inside for protection as long as it's not thicker than 1/4" and the envelope DOES NOT have a metal clasp then it should mail regular first class mail with a non-machineable surcharge if domestic. This might work for other things too.