The Application Programming Interface (API) is the source code interface that the operating system or its low-level libraries offer to support applications from application software or computer programs. This achieves efficiency and speed at the adaptation of new software technologies.
Yes, it is possible. It is first necessary to specify what additional features you would like to integrate into the site, whether it is possible to integrate them and after a final judgment on my part, to form a price for the changes made.
The price is individual and depends on site size, complexity, number and type of items, desired functionality, whether finished or programmed, etc. We can only give you a price after a conversation with you.