Get offcuts of Gatorboard/foamcore (most frameshops have heaps of the stuff, also great for making wall titles) and sandwich the mounted print between tissue paper in that, then a second later of either stiff cardboard or more foamcore and maybe bubblewrap over all of it (to cushion the inevitable blows). If the print is recessed because of the mount, cut out a piece of bubble wrap and put that in the recess before final packaging. Tape all edges really well. Insure the item and post.
There is really no fail-safe method as all items going through the post will be subject to some banging and loading pressures even end up under a tonne of other stuff, including pointy boxes etc. It's a good thing you're not shipping glass framed prints: these are disastrous shipped anywhere except by specialist road couriers who transport and insure fine art.