SetExpressCheckout indicates to PayPal that you are using Express Checkout to obtain payment from your customer.

PayPal will return a token that identifies this transaction. That token will be appended to the PayPal login URL and the next step will redirect the browser to that URL.
Billing Agreements (optional -- this demo supports only RecurringPayments Billing Type)

Billing Type Agreement Description Payment Type Custom Field

Shopping Cart
Item Quantity Price
Add or Remove Line Items (shipping/handling for total calculation only)
Number Name Description Amount Tax Shipping Handling Length Width Height Unit Weight Unit Category