Different unpopular suggestions to follow:
I think that some OPA members use it's services a lot more than others.
Perhaps the occasional players who want to shell out $25 for a little insurance don't need to pay as much as high rolling players who repeatedly deposit $5k with dodgy sites and then ask the OPA to bail them out.
I'm an Auto Club member, and have often used their roadside assistance (usually because I left my lights on and need a jump -- oops). But I only get a limited number of free service calls before they start charging me.
I don't know what form it should take, but I think that some sort of pay-per-use strategy could be appropriate.
E.g. $10/complaint after the first 3/year. (Adust fee and number to taste.)
Also, some sort of commission for payment resolution might be appropriate. If the OPA gets a member paid $1000 that would not have been achieved through personal dealing with the casino, perhaps a $50 or more commission would be reasonable. E.g. 5% of any amount over $100, min $5, max $100. Perhaps this could be voluntary, labelled as a "suggested contribution".
Charging some sort of fees for use could also add some incentive to attempt to resolve problems personally before calling in the OPA, which doesn't always seem to be true of some OPA complaints. And also cut down on the complaints by players who didn't get the $10 they expected from opening an account without playing.
I don't know how best to handle the above suggestions in a fair manner, but it seems reasonable that players who make disproportionate use of the OPA's services should cover a larger fraction of the expenses.