eBay currently has a flash sale where you get $15 off purchases of $75 or more. Add 4 $20 Steam Wallet cards to your cart from Best Buy's eBay store and checkout with the code CPREP4SCHOOL16. Must use Paypal as your payment method. Shipping is free or you can choose to pick up in-store.
Other items are eligible to use the code on so here are the terms and conditions.