I like to go to the horses mouth so to speak. That means looking up a local box
wholesaler in the neighborhood. I have been using cartons that were designed for calender sales. I got them in several sizes and they will work with up to 11x14 prints in the right size. I mount the print, put in an overmat with a slipsheet and some bubble wrap to fill it up. Have sent prints around the world this way with no losses. There is actually a number for these containers. A box website may give you the info you need.
Best, Peter Schrager
Let me speak from experience, Peter's shipping methods work wonderfully.
Let's see what I've got in the magic trash can for Mateo!