This might be targeted, so check your amazon account if you are a amazon prime member and your default card is anything other than a discover card.
Just by setting your default card to discover, amazon will apply a $10 credit to your account which can be used right away. You obviously don't have to use the discover card to pay for your orders, you can always chose another card at check out.
It is a free $10 for a couple of clicks in my opinion. You can go back tomorrow and change your default credit card back to whatever you were using.