If your sales can cover your fees, the fees are taken from your sales. If not, Amazon charges them to your credit card. They might also offer an option to withdraw fees from your connected checking account, but they never did that with me. It was always deducted from sales first, then credit card if sales didn’t cover fees. Here’s more information about all of Amazon’s seller fees and costs if you need more info about the process of selling on Amazon.
Good luck in your Amazon ventures!