From help page

How do I pay for my ads?

When you advertise, Amazon charges for the number of clicks you receive on your ads. Your selling partner invoice will indicate which charges are for advertising costs. You’ll find these charges in the Selling Fees section of your invoice. By default, your advertising fees are deducted from your selling account balance. The total advertising fee is deducted from your selling partner account during an invoice period. In the event that your selling partner account does not have enough funds to cover the cost of advertising, the remaining balance will be charged to the credit card you used to register for Selling on Amazon. If we can’t charge your credit card after multiple attempts, your advertising account will be placed into payment failure.

You do have the option to switch your payment method to deduct from a credit card. Each time your credit card is charged, you receive an invoice. The invoice date is your monthly invoice date, or the date when your credit level changes.

Your credit card is charged when you accumulate your first £1.00 worth of clicks. (This transaction is used for validation of your credit card).

You accumulate additional clicks that meet or exceed your credit limit. Your credit limit increases according to a “credit ladder.” Each time you reach your current credit limit and you make a successful payment, your credit limit increases one level in the credit ladder: to £50, £150, £200, £350, and finally, to £500.

You have a balance due for the previous month, regardless of your credit limit. You incur this charge on the 3rd day of each month.

Note: Your credit card is charged each time you reach your current credit limit. Whenever you move up the credit ladder and your credit limit changes, your credit card charge will occur more than once in that month. If you do not exceed your credit limit, you are charged only once in that month.