They advertise free shipping for orders over $100 on their website. If it works, you'd be much better off buying something for the extra cash, pushing the order over $100, and getting shipping for free. It'd be like getting that $40 worth of stuff for free...
Edit - I just tried it out, and the free shipping deal does work, if you push the total over $100, so if you have some additional shopping to do, it might be worthwhile.