Paying someone to be a middleman on my volunteer charity work feels pretty gross.

Why? They are enabling you to have opportunities to help people that you would not otherwise have. I would consider that a service worth paying for (and I assume I'm not alone...)

It feels like being taken advantage of. It's taking the worst part of crowdsourcing, which is that the majority of the gains are concentrated in a few people while many people contribute to success, but receive a pittance, and making it even worse.

I'm not convinced that the raw economic incentives work out, either. I have tons of opportunities to help people. Supply of people who need help greatly exceeds my ability to provide it. I can't imagine paying for the privilege.

If they are providing a valuable service, this will lead to competition and more vendors who want to help you provide your charity more easily/efficiently/augmented/whatever.

If they are not providing a valuable service, they will go out of business.

I think this is a good idea. The subscription could potentially be done similarly to Humble Bundle where a decent percentage could be ring fenced for charitable/non-profit organisations. In this case you'd expect it to go to organisations helping the bodily impaired.