Hi,

to display this information within the shopping cart, you can modify the following controls and show your "savings":

\CMSModules\Ecommerce\Controls\ShoppingCart\ShoppingCartPreview.ascx
\CMSModules\Ecommerce\Controls\ShoppingCart\ShoppingCartContent.ascx

for the invoice and email notifications please edit templates in:

CMS Site Manager -> Administration -> E-mail templates -> your E-commerce notifications
CMS Desk -> E-commerce -> Configuration -> Invoice

You will probably need to create a custom macro for your logic.

If you also want to display some more information in the list of products within these templates, you can edit the transformations in:

CMS Site Manager -> Development -> Document types ->  E-commerce - Transformations

Dave