Checkouts API let you finalize the order by creating a payable unit with proper user details, taxes handling and all the data that is needed to fulfil the order properly. Most likely it is enough to use Storefront's Checkout Components that take the burden of implementing proper API interactions yet giving the full customization powers and the native feel.
Real-time stocks in checkout
Similarly to shopping cart, up until the user pays for the product, the stock unit will not be reserved. That's why during the checkout phase Tipser is taking care of live product stock checks and price monitoring to make sure that the user get's an early alert if the product is getting out of stock or the product's price have changed and if such a modification will happen, the proper metadata will be returned in the response payload so the user can be alarmed.