I mean, if you genuinely want to rip off the broker on a technicality, that's up to you.
Presumably, when you listed you knew there was a comission (and knew the rate) and would have expected it on the sale. If it were me, I'd sign the addendum, because I agreed to it informally earlier, and it would strike me as unethical to take advantage of an honest mistake.
But it is not my money, so you might choose to decide that my opinion is none of your business. If you want to get an opinion on what is legally required, you'll need to talk to counsel. IANAL.