Flat fee MLS broker 0.5% due at closing (Colorado minimum service state)

[Braath Waate]

After some research I've learned that most flat fee MLS listing services charge an additional 0.5% due at closing in Colorado (specifically, Grand Junction). What I couldn't find out is how that closing fee is enforced. Do they require signing a Exclusive Right To Sell contract? If not, and I find a buyer and close the transaction myself with my favorite title company, would I have to pay that fee?

As an experiment, I set up a listing with houzeo, and got to the point where they want my credit card, but they didn't require signing any contract up to that point.  So I'm mystified as to how this 0.5% closing fee is enforced.