Don't know what you mean by "small back window", may be back windshield or crew side window, but I would cut a piece of 3/8" plywood 1" bigger all around than the piece of glass. Put half a dozen dots of silicone evenly spaced around the edge of the glass and place it on the plywood and let it cure.
Wrap this in bubblewrap, wrap that in cardboard. Mark the cardboard "FRAGILE GLASS".
Take it to UPS or FEDEX. Insure it for the value you are selling it for, and away it goes.
I would do this only if it was a piece of glass where originality was important. Flat glass is pretty inexpensive.
If I was buying, the price of the piece, and the shipping and insurance would most likely be the same or more as buying new flat glass.
Hope this helps.
I shipped some vent glass and what I did was sandwich them in cardboard, then, I put bubble wrap all around them. Then, I put them in a box with more bubble wrap. There was no room for movement. These went through the US Mail without problems. If you use plywood, that will raise the price of shipping way way up. I know that the small back glass isn't that big, so you should be able to find boxes for it. I also like bubble wrap better than the styrofoam peanuts. They seem to "move" around.
So, like the artfulldodger said, wrap it in bubblewrap, and mark the box with FRAGILE all over it. I just wouldn't use plywood due to the weight.
That rear window glass is tempered and is pretty durable. It won't crack like a windshield, but it will break into a million pieces. But, it is strong, and will take quite a bit of abuse.