Develop Automatic-Checkout for a Store Using HTTP Requests in C#

I have developed a bot for a certain store that adds a desired product to cart, however I can not figure out how to write the automatic checkout for it. Using a "checkout preset" structure that is passed, you must be able to fully checkout with the preset's details automatically in the site. It must be in a readable format so that I can integrate it with my program. Please have experience in how HTTP web requests work.