Thank you for your inquiries. Let me see if I can shed some light on your questions.
Processors including PaypalExpress checkout which requires customers to leave the website are currently not supported. We are currently looking to intergrate Paypal Express checkout by the middle of this year. We support 3 payment providers at the moment including Authorize.Net AIM, PayPal Payflow Pro and SagePay Direct. I have included a blog post that goes into further detail about the Payment Provider model
The custom payment provider model only allows you to implement providers who have a transparent API. If you want to achieve this functionality now, the only option is to have your own custom checkout widget, which captures all order related information and talks to the payment processor.
I hope this information has been helpful. Please let me know if you have anymore questions.
the Telerik team
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