My customer give me this link https://www.paypal.com/webapps/mpp/compare-business-products and want me to implement feature \"Customers pay without ever leaving your website\" w
Payments Advanced is a separate product from Express Checkout. It's part of the PayFlow API, so you need to have an account setup at http://manager.paypal.com. If you don't already have this then you're probably on a "regular" PayPal account, and you would indeed need to get them to switch you over to PayFlow. Simply applying for Payments Advanced should trigger that action for you, and then they'll send you login credentials for your PayPal Manager.
With Payments Advanced you make an API to obtain a secure token from the PayPal server on-the-fly, and then you use that token within some basic HTML code similar to a standard button.
This causes the full checkout form to be displayed within an i-frame on your page. When the user checks out there they'll never leave your site.
There's a good 2-part video on YouTube that covers this in more detail.
Part 1 | Part 2
You can also find plenty of documentation about it at X.com.