On Amazon, they allow you to apply any unused balance to an existing order. When you check that they say that you have a loan of $ 150 in your account, you would like to apply this to your order, by default it is given in a check mark.
Its quite neat and simple, it does not allow you to apply part of the amount from what I saw.
Then, when you proceed to pay, you pay $ total - balance.
So, if you have only 200 dollars, credit card payment will be for 50 dollars.
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