for customer invoices and receipts, allow fields to be done via a mail merge in Microsoft Word

We have a lot of customers that contact us by phone to make payments by credit card for their orders. They require a receipt, which they want via email. We would like to be able to customize receipts to include fields that are not currently on the receipts, and wew would like the ability to customize the receipts to only include information we want to share with the customer, and add customer or content specific information we wish to add to the receipts.