The Purchase Details section is a placeholder. It represents what type of data will be printed there.
When editing the Thank You Message, this section cannot/should not be edited. The section can be removed, if desired.
With the Purchase Details section inserted, the Thank You Message email will reflect all of the purchases made by the donor - the quantity of the levels and add-ons purchased, the item name, and the total cost for the item. Below that section is the Purchase Details totals which is editable. You can also remove a line, such as Coupon Discount Amount, if that does not apply to the event.