Amazon Pay is Amazon’s payment product which is similar to Paypal or Google Pay. There are running an offer to promote it at the moment.
You could check the offer details here. Pay for a purchase using Amazon Pay within the next seven days and a £5 Amazon voucher will be deposited in your account.
Obviously it’s a niche product so not widely accepted by many online merchants yet. Thanks to @Azwipe on hotukdeals, it appears that you can buy an online game for only 10 eurocents here.