I like the way I've received prints shipped from West Coast Imaging. They put an archival interleaving sheet over the surface of the print and put the sheet and the print into an archival sleeve and then tape that sleeve to an oversized sheet of corrugated cardboard with drafting tape, and then package it with two sheets of the same corrugated cardboard on top of the print, two under (four sheets total, including the sheet that the print is taped to), and box the whole package in a corrugated cardboard box cut precisely to size. If you've sent them disks, negs, or transparencies, they put them between two of the sheets of cardboard other than the ones directly protecting the print, also in protective sleeves, taped to the cardboard sheet.