Hi, it sounds as if you have already checked out the automated standard system options for " * Letter Generation" - although I must admit it has been years since I have had to look at it.
The BROOMS system also offers - "Chaser Sequence Maintenance" for chasing - although by letter is maybe a tad old fashioned these days.
An alternative method - You can also generate letters when setting up the transaction with a Date to be Sent ( print date in advance) and just print all unprinted letters on a daily basis.