It depends on what kind of API you have purchased. Mostly the provider will give you the sample to integrate in ASP.NET. You may have to integrate payment gateway as well to get the payment from users.
If you can provide more details about the API we may be able to help you.
Asheej T K
if you do not have any then lease contact the vendor of the API.
You may try searching some code in the net/ google it.
Thanks & Regards
Anil Kumar Pandey
Microsoft MVP, DNS MVM
Mostly who provide API they also given integrated code so you can search where you purchase API
You can go through Below links
Name : Dotnet Developer-2015
Email Id : email@example.com
'Not by might nor by power, but by my Spirit,' says the LORD Almighty.