Actually such services already exist for running, triathlon and bike races. One of them is active.com and the other one I know of is bikereg.com. Competitors pay a transaction fee of around $2 - $5 per event. It’s very beneficial to competitors that are trying to enter an event that sells out on the first day that entries are accepted. (I guess events selling out is a concept foreign to chess players.) I don’t know whether they charge the race organizer or whether the profit is derived from the transaction fees.