Commerce: How to skip checkout cart and redirect to PayPal
It's not possible without custom code. The customer either initiates the Express Checkout payment by clicking the button form on the cart page, or the Commerce PayPal module does it on a form submission when the customer chooses PayPal as a payment option on the checkout form. That requires sending an API request to initiate the payment and get the tokens that must be passed with the customer to PayPal to submit payment. With custom code you could simulate the customer choosing PayPal as a payment option on the checkout form and initiate the redirect, but there's nothing that exists to do that that I've seen.