How do you manipulate the email receipt?

I want to show a discount or a fee on an email receipt. How do I access the email portion of the receipt so that I can add this item on when an email receipt is sent?

Right now when an email receipt is viewed, it only shows the “subtotal”, “tip”, and “total paid” on the receipt.

When the receipt is printed by using the PrintedReceipt api, the fee and discount is shown on the receipt.