Hi,

To be honest, I am not completely sure what you would like to change, especially, if you only need to change the way Kentico CMS display the total price in the payment step or you need to change the way Kentico CMS sends information about the price to the paypal.

In the first case, you can simply modify the following page: ~\CMSModules\Ecommerce\Controls\ShoppingCart\ShoppingCartPaymentGateway.ascx which displays the payment step for the PayPal.

On the other hand, if you need to change the PayPal functionality, you would need to either create a custom PayPal provider by inheriting from the existing one and overriding method you need or you could create a custom PayPal provider from scratch.

This webinar could give you more information: Developing Custom Payment Gateway

Best regards,
Michal Legen