PayPal确认然后返回一个字符串HTML（js..或其他任何...）

So i'm looking to make a website that you can use a PayPal "Buy Now" button, it will check to see if you've actually paid and then return a string, in this case, a Steam Game Key. Obviously I don't want to give them out for free... So i need a way to only give out 1 per payment for that thing. Lets say I have 1 item for . Someone buys it and they get a key.... how do I confirm that they've paid and then give them only 1 key from a list of them.... then remove that to not give it to anyone else.So i'm looking to make a website that you can u