Thanks!
Are you able to pay for orders online with more than one credit card?
There are a few payment methods you can choose on the checkout page and you can combine them, but you can only enter one credit card number. For example, if you buy a $150 item you can pay $25 with a gift card, $25 with a Staples rebate card and $100 with your AmEx.
That said, if you want to get the rebate for buying MasterCards, you have to buy in store. If you were asking about buying other items online, that's OT.