Coding to merge fields filled in by customer online to merge into a word document

I normally use Interspire to build simple e-commerce sites for my clients, however, this one has a slightly unusual request and I'm not sure of how to do it. I don't have any coding knowledge myself.

The client would like people to be able to purchase pre-written letters written by him, and the fields that the customer fills in will merge into the appropriate place in the word document which should then get emailed to them once they have paid. The client will need to be able to stipulate different merge fields in the word document in future as he will be creating up to 150 of them.