How do I compose a commission contract that one will be getting 3% on any sales of the construction material he introduces to the store?

What is the right form and the right wording about it?

Thank you so much!

Are you saying that you will be the one that has to be providing a contract for what you are asking? Who is "he", please? What do you mean by "introduce?" Not sure exactly what you are asking about here. To be sure in things of this sort, it's probably best to be consulting the advice of an attorney. But, we might be able to help you here if we know more specifically about what you are asking. Thanks!