No matter how well you pack everything, and even if you crate it, it might get damaged or completely lost. Make sure it is properly insured and try to leave adequate time to ship replacements if necessary.
Express shipments such as 3-Day Express or 2nd day air are handled less and treated a little more carefully than ground shipments.
For prints alone, the most economical is tubes and the safest method is PVC or PVC like tubing.
Acrylic is MUCH safer than glass. We only have acrylic damaged about once a year (and I am not sure how they manage to do it). When shipping glass make sure it is a few inches from all 6 sides of the box, the glass can not be allowed to flex, bind or vibrate against other items in the box.
If you are shipping expensive picture frames and/or artwork consider crating. It will significantly reduce the damage rate due to its strength and due to the fact that is harder to throw a crate. It can possibly still be pushed out of the back of a truck or lost so you still need to have proper insurance.
When we ship large glass we only do so for large frame orders so we can put it on a pallet and ship it freight. A heavy pallet is being moved by pallet mover instead of being thrown.
When we pack picture frames we place something between them such as foam or cardboard and use stretch film to secure them tightly to each other. The box will be vibrated and shaken for many hours so you want to make sure that the contents of the box do not damage each other. We take the resulting stack and wrap it with 1/2 bubble on all 6 sides. We then place this in a box that is couple inches larger on all 4 of the smaller or edge sides. We fill the remaining space with bubble, craft paper, or cardboard. What is used for void fill is not critical. It is critical that the void fill not compress overtime allowing the contents to start slamming around in the box. It is also critical that the contents can’t shift around the void fill and get next to the edge of the box. For that reason we do not use peanuts.