http://www.doctorofcredit.com/paypal-digital-gifts-limiting-us-4-gift-cards-per-10-days/
Doesn't seem to apply to you but I get that error when the giftcard has been used before on another paypal account.
i think it might have something to do with my husbands ebay/paypal account somehow being confused with mine.I got that error a bunch this past week. I concluded that in one account, it was because the account hadn't yet made any large gc orders (finally worked with an order under $200). In the others it was because they are at the $5k/6 mo limit. I wasn't ordering from ppdg, but if you are, look at what @akoichig said too.