Aside from the automatic offer discount stated on Automatic discount on upsells, you may generalize the discount to every product for all placements: Vanga Cart, Post-Purchase Page, and Thank You Page.

You may customize the automatic offer discount for every placement. For example: on Vanga cart you may set the discount to 10% while on the Post-Purchase and Thank You page were 15%:

Note: You can only use this feature on the Pro plan only