Might try the tube route next time. I sent my prints in stiff envelopes sandwiched between illustration board, plus they were 11x14. Between the size and the weight it cost almost 10 usd each to ship. I choose priority since it was only a dollar more than parcel post. To contrast I receive 2 prints including a 16x20 in a mailing tube and the postage was under 3 usd. The mailing tube seemed quite robust.
If only we could discover these things the first round