I have the same case. Maybe desable the some "Requerid Fields" like address, fone, Zip Code, phone... I'm using "Paypal Express checkout" and the first CHECKOUT apear Proceed to Checkout button and bellow apear also the Checkout Express Paypal button. If the costumer click in the second button, go direct to Paypal paiment. So I think that if hide of Prestashop button it work normal. I do not know. You can active Express Paypal in Modules