PayPal Express Checkout
From WHMCS Documentation
The PayPal Express checkout module allows your clients to check out in a single click without the need to enter their details into the WHMCS order form. The client's name, address, email and phone number will be taken from those stored in their PayPal account and a one-time payment sent when the Express Checkout button is clicked on the order form.
Begin by activating the payment gateway under Setup > Payments > Payment Gateways. Next enter your API details, instructions for obtaining these can be found on the main PayPal page.
Express Checkout is only used to make the initial payment via the order form, in order to receive further payments directly via the invoice another payment gateway is necessary such as the standard PayPal module.