About 10 years ago I worked for a startup that used a sales agency. Unfortunately it did not go well, in the nearly 3 years I was there they never made a single sale.

They didn't understand our product and had no desire to understand our product. I advocated to end the contract with them, but management refused.

So I think I would want some verbiage in the contract that says I can just walk away if I don't think it is working out. I understand you need to protect your interest too, but I wouldn't want to get strung along by someone who is just not willing to put in the work.