A simple stamp on the paid invoice is all I use. I was told by an attorney that is all that is necessary and is as good as any long document. Anything can be challenged and sometime less is more when you are defending it.
I printed up something that I print as needed. Something to the effect of: The individual picking up the framed item is authorized by the owner of the item to transport it to them, and acknowledges the item was received in good condition
(this type of thing may protect you, should they put a ding on the frame and blame you)