Pro Site Checkout Issue for new user signup

Non-Admin users can't checkout. This justifies the "Checkout Permission" settings in Pro Sites settings. But new users are supposed to be able to sign up for a pro site and pay, right? If yes, why that's not working on my site? When a non-admin user tries to purchase a pro site and clicks the PayPal payment button, the checkout page is simply reloaded. But when any existing admin user tries, he is taken to paypal.com for payment. What am I missing?