I have no clue how commercial account reps are paid in this industry. I am sure you have to set out some standards on estimating and such. That is why a percentage of gross is scary to me. Any wing nut can sell products and services at a loss to the company.
Net is much safer but they may object to how the account is managed. If you are paying salary plus commission then that is one rate, straight commission is another. You also need a non-compete/ no raid provision.