I get paid at closing about half of the time. I must admit that I have not had a bad experience. I don't charge extra either.

All I do is keep a simple Excel sheet with the address, buyer's name, and realtor's name and number. If I don't get paid when I am told, I call the realtor. It has not been a big deal for me. Actually, my realtors like it because their client does not have to come up with my fee up front. It makes them happy, so they call me more often. Seems like a win-win to me.