How can I offer a discount for another related product when a user purchases a product?

One way to do this would be to set up the discount code and include that code in the Product price additional product details field.
This will display both on the Order Confirmation email and on the confirmation page that is viewed upon completion of the order.


First, set up the discount code on the related product. For more details on setting up discount codes, check the online help here

Next, edit the Product Price and update the Additional Product details field to include the Discount code you have set up.
Be sure to click the Update Price button to save your changes.