I have been full time WOHM and am now a part-time working at home mum and I have to say, I have, in my former life, felt like to original poster felt aggreived at what seems like a constant stream of requests and demands.
I think as a WOHM you often feel a bit ostracised from the school anyway, and particularly from the full-on PTA-type mums, and let yourself fall into the trap of thinking 'if I don't put my own time/ blood sweat and tears into a cake then somehow my DS/DD will be emotionally scarred for life'... They won't!
I am now a more active member of the PTA, but I still get rather cynical at times. We recently evaluated a Spring Fair at which the PTA raised about £3,000 after many months and man-hours of preparation. I pointed out that this equated to £8.80 per pupil, and the same outcome could have been achieved by increasing the annual 'school fund donation from £40 to £45, and adding £1 to school uniform costs (which were already very low compared to typical uniform prices).
As a working parent, I know which one I would have preferred, but I appreciate not everyone would feel the same, and be in a position to contribute more.
You have to decide for yourself what is a reasonable commitment for you in time/ money and then just stick to it.
And if you've contributed a lot recently, or in the past, don't beat yourself up about it if you want to pass on the requests now.