When the FBA seller fails to sell anything and can't pay the storage fees, the seller has a balance. If the filed card is valid, it becomes a debt, and Amazon will seek to get payment on this debt. Amazon will take this from your account, rather than mailing you a check or letting you pay via credit card.
Amazon charges a fee for the sold item with the attached coupon. When the buyer clips the coupon and makes the purchase you pay $0.60 as a fee for their “coupon service”.