Hi supun.d.kaduw...,
You can use PayHere with ASP.Net.
For payments you can use the PayHere Checkout API. It is just a simple HTML Form and Backend POST endpoint integration.
For your reference, the Checkout API documentation:
https://support.payhere.lk/api-&-mobile-sdk/payhere-checkout
For more details refer below links.
https://docplayer.net/41670165-Payhere-e-commerce-checkout-page.html
return_url - This is the URL to where your user is redirected after successful payment. Should be a front end URL.
cancel_url - The user is redirected to this URL if the payment window is closed.
notify_url - After a successful payment, a POST request is sent to this URL. This is where you can get the full payment information. This should be a back end URL and should be publicly accessible.