f1ffb1c8____Technical_Overview___PayPal__Description
[Step] 3b [Description] 
Integration Point 3 When your customer places the order, you make an API call to PayPal to request payment. (Your customer does not see this step.) The payment transaction is initiated, and PayPal sends your customer an email receipt for the payment.
PayPal Express Checkout - Technical Overview - PayPal
Description