PayPal 

The PayPal Core SDK addresses the essential needs of the PayPal API caller: Frequent and up-to-date releases: The PayPal Core SDK is available on NuGet, which translates as immediate SDK refreshes upon updates... More information

The PayPal Merchant SDK provides the following: SetExpressCheckout API Operation (NVP/SOAP): The SetExpressCheckout API operation initiates an Express Checkout transaction. DoDirectPayment API Operation... More information