I have created templates in MS Word for letters that we send to customers.
These contain "Text Form Fields" so that all a user needs to do is TAB
through the document and fill in the necessary fields (Customer name,
address, etc.) and the rest of the document's content is locked/not editable.

I am looking for a way to bring this sort of form into Outlook so that a
user can simply change the contents of a few fields and send it to a
customer. What this email serves as is a way for our customers to read over
their order and Confirm the order. Right now, we have our customers hit reply
and type "I authorize this order..." into their response. Is there another
way where I can add a checkbox for the customer to check, authorizing the
order, and then send it back to us?