I am one of the few who has created a custom letter for every individual client.  Been doing it for years.  I created my own template and then add or subtract info as each return warrants.  Clients really do appreciate non-boilerplate instruction letters.

The process is laborious in ProSeries. Some things work really well (like picking up client bank account name, routing number and account number for verification) and some are a pain in the ass (like using tabs for spacing: Maryland fits fine but District of Columbia throws the tabs off as it's too long).  

Play with it. Contact me back and I can send you a sample template but expect to spend a good 2-3 hours getting it to your liking.  And then test all scenarios (balance due e-file or non-efile, same with refund, or no tax due or refund at all).