That's sort of it, Bob...
In this case the organizer didn't bother asking Jim if he could do all the shows, they just *assumed* he would do them, and he's already booked on some of the dates. Jim doesn't want to half-ass those gigs by sending what the organizer asked for - a speaker and a mic.
Since Jim seems to heavily discount or freebee these gigs for the charity *when he's available* I think the charity person needs to hire, at full price, a comparable replacement for Jim. That's why I said walk away - they're treating him like a hired vendor and he's doing it as a volunteer. That's also why I suggested Jim ask if the organization would like a referral to another provider for the events he cannot help with.
I don't think anyone suggested he perform a less than appropriate service no matter what the circumstances.
I agree with you 100% Tim, and my statement was not directional, just my statement. Jim is realizing the fruits of his labor and experiencing the all too common "I've been taken for granted syndrome". The customer is much too comfortable with their current services, and Jim, by doing a good job for little or nothing, has now inadvertently led the customer into believing it's all well and fine to put demands on Jim. He needs to cut them loose for a few gigs at least until they begin to appreciate his talent, and his generosity.