The glass in the frame is slightly smaller than the ledge that supports it. You will see the glass move slightly when you clean it. If the box that the frame is in is suddenly jarred it could cause the glass to bang into the wood frame and chip of crack. Even moving that small amount of distance will a sudden jolt can break the glass, especially vintage glass that was made before safety standards.
You first need to immobilize the glass as much as possible. First cut a sheet of 6 mil or similar thick plastic large enough completely wrap the frame. Lay the plastic of a flat surface and place frame face up on top. Now take a second piece of plastic just large enough to cover the front and extend past the four edges several inches. Lay this smaller piece of plastic across the front of the frame and center it.
Create some cardboard panels the same size as the glass and place them on top of the plastic on top of the glass. Add panels until the cardboard is slightly taller than the outside frame. You have two options with the cardboard panels. You can have all the cardboard panels the same size as the glass. Alternatively add two the same size as the glass, with the rest being as as wide as the glass but not as tall and running across the horizontal center of the lower panels (this will save some weight).
Now wrap that larger sheet around the frame tightly and tape it. The idea is that the tape will compress the cardboard panels downward, compressing the frame components together, thus making it less likely for the glass to move at all during shipment. The glass panel will now have some tensions against it to hold it in place - similar to grabbing the frame with you hand and squeezing your hand together. The cardboard also protects the glass.
Now enclose the frame in 3/16" diameter bubblewrap. Take apart a large box and wrap it around the frame on all sides as a inner cardboard sleeve. Now find or create a box that that will provide a minimum of 3" of empty space on all sides, top and bottom into which to place crumbled wads of newspapers. Cover the entire surface of outer box with tape - its cheap insurance.
The boxes are free from local stores. The newspapers are free from from your own home, family and friends. Bubblewrap is around $40 for 700 feet shipped free. I use tape that is less than $1 a roll.
I package objects while watching TV so time flies on by. With three decades of experience its mostly muscle memory at this point.
In nearly 25 years selling online and additional years working for warehousing facilities I've only had one damaged package as that was done with a multi ton UPS forklift, but the buyer kept the item as the retail product box has a gash in it but the item was not damaged.
