Customization of the behavior of the CMS application or its specific module according to your exact requirements is described in detail in the Custom providers chapter in the API programming and Kentico CMS internals section of the Developer's Guide.

 

As this type of customization is available also for the E-commerce module, you can override the default behavior and calculations by using custom providers that ensure various operations.

 

E-commerce module customization samples can be found in <web project folder>\App_Code\CMSModules\Ecommerce\Samples (or in <web project folder>\Old_App_Code\CMSModules\Ecommerce\Samples if you installed Kentico CMS as a web application).