If it isn't obvious (they usually state it in rather large print near the top) then check to see what they require for a receipt. If it says something like original register receipt you will might have issues using a packing invoice/receipt from an online place.
Also there might be exclusions saying they won't pay you back more than the purchase price (so if there is $20 rebate on something and you bought it for $15, you might only get a $15 check).
But everything should be in the fine print. So just keep copies of everything so if there is a problem you can call and complain