I made my own...it needs fine tuning. Basically it takes a form letter, lets you pick who to send it to. Then it can either print all the same letter to all the people in the found set, or build each letter, so you can customize each one. Then after printing it gives you the option to put a either a copy of the letter for each customer (separate related table) or just that a certain type of letter was sent on a certain day.
For example, for newsletters, in the Customer's contact table it will just say:
10/15/15 - fall newsletter
But for something more important:
10/15/15 - late invoice notice - then a box with a container field with the letter saved as a PDF.
When I get a chance I could strip it down to just the essentials and post it.
You can also google FileMaker Letter Writing. I think I saw a module someone built (maybe FM Starting Point), plus there are videos on youtube.
The one I built was based on the VTC FileMaker 12 video training by John Mark Osborne.