I have one data point for you on this. I had some Sears gift cards from various source with which I purchased a power washer from Sears through the NW shopping site with in-store pickup. The GC's almost covered the whole price but still had to pay $30 or $40 as I recall. When the miles posted I was given credit for the entire purchase not just the amount paid with the credit card. Seems when they track it by the order and the total of that order (after discounts but before any payments). Thus I would think it would be possible to buy the gc's then go back and buy what you want later and get miles both times. As always, that is just one instance I have and YMMV.