5loth: I would say the are the same thing.
Given the FTA only specifies for the term Rain Check, and you have done exactly what a rain check involves, if you went to the DT the Rain Check rules would apply.
Unless you can show me the laws involving a 'back order'?
Where exactly does it use that term? I cannot find it anywhere in the text of the Act itself (unless it is in one of the amendments, but I cannot be bothered finding it in there)