I believe the gift certificate says, for only in-stores only. What you can do is... go in and use their in-store computeres, buy what you like, enter the coupon code, and at the end, say that you want to pay at the cashier. There'll be a printout of your order, you can take that to the cashier and you should be fine.
I was able to do this about 3 months ago, I don't know if their rules have changed. I know Staples's becoming aware of all the coupons out on the internet.