How to change Receipt body text and mail merge fields for Receipt/Letter Template
From the Letter object, click on the Letter Builder Button.
In the list of available fields, locate the field you wish to merge in the receipt body. RIGHT-CLICK, then copy.
In the Rich Text Field for the receipt body, place your cursor where you would like the merged field to go, then RIGHT-CLICK and select paste.
After making all your edits, press SAVE button.