Template, invoice, possible?

I was wondering if its possible(and if so, how would you go about it) to make a template text file and have some parts of the template editable, so I can type new information into the template, and then when its done it saves the page as a US Letter size text document with the permanent aspects of the template and the changed aspects as one.

I am trying to make a billing invoice software with max but don't know if I am better using Act! or something else.