Ask HN: How do I make my HOA to make their online payment page secure?

Go to a meeting and talk to them. They may not have the expertise to even understand the security issues. Don't forget, your HOA board is your neighbors, so helping out is the best way to get things done.

They may be in violation of the ToS with their merchant transaction processor, which is a bit of a self-solving problem if they get caught.

Steal their credit cards. Just kidding, don't really do this. (Do it).

Send them a check?