nopCommerce site: https://www.nopcommerce.com
Listing on nopCommerce "extensions and themes" catalog: https://www.nopcommerce.com/p/1149/paypal-express-payment-plugin.aspx
PayPal site: https://www.paypal.com
PayPal Express: [https://developer.paypal.com/docs/classic/express-checkout/gs_expresscheckout/](Express Checkout API Getting Started Guide)