> All transactions happen in an overlayer modal box, users will not need to leave your site during the whole process unlike other methods, where users will be redirected to PayPal official site to make payment.

I've always used ExpressCheckout with a redirect to paypal.com. I've previously have set up a popup and an iframe versions of a PayPal checkout. But I think the ExpressCheckout docs explain the flow with a redirect.

Could you point to the relevant PayPal documentation? Thanks!