VH, you wrote that you are often accompanying these choirs because 'there is no one else stupid enough to do it'. The thing is you have obliged by playing repertoire not designed for organ accompaniment and no doubt done it very well, which is why they ask you to do it again. This has involved a great deal of work on your part rearranging this stuff so that it will work for them, and they obviously have no idea of the skill involved and so don't think to reward you appropriately. (Choirs have a mindset that you love to play for them just as much as they love to sing, which in my limited experience is somewhat wide of the mark, although my choral accompaniment was done with the piano and was unpaid: " ...good experience for you, dear..." palmed off onto me because my piano teacher at the time didn't want anything to do with it.
The only solution that I can see is to get yourself onto their artistic committee and veto anything that is going to involve too much rearranging on your part. At the moment it seems that they decide what they want to sing and then present you with their programme.
It sounds as though more than one choir is involved here so that might be more than you want to take on. The other alternative is to not have a set fee. The trickier the job, the more it would cost them.