Hancocks of Paducah doesn't charge for shipping of back orders at all (only for the item itself). It's one of the reasons I like to deal with them, though it can be frustrating when things arrive in dribs and drabs, and I have been known to forget that something is on back order, and order it again!
I can only presume that they absorb the costs because ... what? They had it in their current catalogue and didn't have it in stock, ergo they regard it as their fault? They want my custom? They are willing to provide this service to keep my custom? I don't know what their thinking on this subject is, but whatever, it works for me.