In Amazon's words, "Amazon Pay is a service that lets you use the payment methods already associated with your Amazon account to make payments for goods or services on third-party websites."
In other words, you don't need to type in your payment information again, and it's more secure because Prestozon doesn't see or store your payment information. You can also manage your payment methods or subscription right from Amazon.
You don't need an Amazon Pay account - as long as you have a credit or debit card associated with your Amazon Seller account, you're go to go. Amazon will automatically offer you all your cards as options when you check out from Prestozon.
Here is an article from Amazon explaining Amazon Pay.