Are you thinking of writing a bot yourself? Personally, I would avoid implementing an automatic payment system at all costs. There are just too many pitfalls especially if it's a card payment (which, reasonably, would require 3DS). It depends on your case and the website that you target of course... But do you really need your bot to pay automatically?
I doubt that Adidas, Nike or any other major website decreases their items stock strictly after a payment has been completed. Doing things this way would result in clients realising that their item is no longer available only after completing the payment. And then what, the website would have to reject the payment or process a refund? That would be a horrible decision both UX-wise and technically.
I would expect them to reserve an item when a customer goes to the final checkout step or when a customer puts the item in the cart. You will have to test this assumption, of course, but that seems reasonable. And at that point all your bot has to do is to put your item in the basket, head on to checkout and you can take over the session manually to complete the payment yourself.
