Ed, I got into that jam with a customer that was buying a new system about every quarter, privately held motel chain. That was OK until they started buying systems from someone else and then wanted help fixing the mess so that the programming was the same as the ones we had put in. That gave me the opening to start charging for every bit of 'phone help'. And when we started charging 2-3 days of 'phone help' on everynew system, they started having us do the installs, even though they were still buying from the same guy, who had been farming out the install to whoever. Then we would not honor any warranty, or give any assistance beyond programming, on the 'other' guy systems.
Anyhow, i'm with the majority, send a letter that outlines why you need to start charging. If the systems are capable of remote, you might offer to do a backup each time you help them with their program changes, before they make the changes, and after they make the changes. John C.

When I was young, I was Liberal. As I aged and wised up, I became Conservative. Now that I'm old, I have settled on Curmudgeon.