I went in today to trade a few items, and asked for clairification on the $5 montly cert (even though I still haven't seen mine). The store associate, and store manager from another local store was there. They both said it could be used on anything in the store (with the exception of 3rd party giftcards (amazon, taco bell, shell, etc.) including new and used, and the manager specifically said you can use it each month to buy PSN/Nintendo Eshop/Xbox cards for $5 after the discount on $10 cards.
This info all seems to different than what I've seen online so who knows.