By default a user who has purchased a product or subscription during the registration process will get access to the site.
If a user doesn’t complete the Stripe checkout process they will be redirected to the Billing page on their Account when they next log in. They will not be able to access other parts of the site until they have purchased. They can purchase a product from the Billing page:
Restricting Content to Specific Products
If you site has multiple products people can register with, but you want to restrict certain content (posts, pages, custom post types) to be only visible to user who have purchase a specific product, like a higher priced product, you can do this with the Content Restriction addon: