I had a similar question with regard to the rebate from Epson for buying a 870/875DC and any digital camera.
I called Epson this morning asked them about the rebate because it said that you needed to send in "cash register receipts". I told them I ordered my camera online and they said that just an email confirmation wouldn't be good enough. The rep asked if I have the packing slip and of course I don't think I still have it so I asked if I could send in the detailed order information from Mercata as well as the UPS tracking information and she said that should be fine.
Hope this helps.
LOD