If you have configured the “Payment Integrations” from the admin side and configure the Payment Settings from the List Builder, only then the portal users will get the Pay Now button for payment. 
By clicking on the Pay Now option, the portal uses will navigate to the Payment Page that is configured from the Page Builder by adding a Payment widget. 
If you select the “PayPal” payment method, you will get the PayPal login credential page in the new tab. 
If you have selected the “Stripe” payment method, the stripe form will appear where they can fill up the card details to make the payment. 
You can back to the customer portal by clicking on the ← Back text caption as shown in the screenshot.