As the owner of Frame Destination (thanks for the mention Taka), I am familiar with shipping frames. Depending on your price point, double wall boxes with foam inserts and cutouts for the frames are excellent, although they are expensive. In our case, we are shipping empty frames, which are much less valuable than those with artwork so we have to keep costs very low.
I would like to point out that no matter what you do, if you ship enough you will have occasional shipping damage. Even if you ship frames in crates it will not stop a forklift fork from going through the box. Our damage rate is less than 1%, but it is not zero.
In general, we center the frame in the box and we like to have a combination of different types of protective layers. We mostly rely on ½ inch bubble.
The front and back of the frame are not damaged very often so we typically just place at least 1 layer of ½inch bubble on those two surfaces. Most of the damage occurs when the box is dropped on a corner, so w try to have 2-3 inches of space between the edge of the box and the sides of the picture frame. We typically add 1-2 inches of bubble followed by crumbled craft paper. For heavier frames, we may add extra layers of cardboard, foamcore, or even plywood around the sides of the box, especially on the corners.
Avoid putting too many large frames in one box, or a lot of glass or acrylic in one box because as the weight of the box goes up, so does the impact force when the box is thrown out of the back of truck and lands on a concrete floor. When there is more than one item inside the box, make sure they are securely attached to each other. The box will receive a lot of vibration during transit causing the components to damage each other. For example, when we ship metal frames with glass we place additional spring clips in the frame to keep the glass from moving around inside the frame.
Very few box companies make the large flat boxes that are convenient for shipping frames. A few make telescopic boxes for mirrors, but Uline is the only company we know that has a good selection of boxes for shipping frames.