It sounds to me what you are describing as a commission is actually a bonus (therefore an incentive.) I also take what you mean "determine the payroll taxes" as federal tax withholdings of employee wages.
For one of the companies I worked for, we had to distinguish between commissions and bonuses (monthly & quarterly incentive pay) in order to insure we were in compliance with Federal and State withholding requirements. Our definition of a commission was as Mr. Brennan defined it - a commission is an income payment based on a percentage (usually of the selling price) of the product or service. In our case, commissions were paid on the amount of the insurance policy that was sold. A bonus for our purposes was a monthly or quarterly incentive based upon achieving certain target levels of policies sold for the month or quarter that employees could earn in addition to their commissions.
The deference between the two has an impact on the tax withholding (payroll taxes) of the employees' pay and therefore on their take home pay. If their payout is categorized as a commission (ordinary income) then their tax withholdings are base upon their expected annual income level.
A bonus on the other hand (supplemental wage) which my be a specific % of monthly objectives, quarter performance metric, etc., will have tax withholdings generally around 25% (flat rate) of the pay out/bonus which is what most companies use do to avoid more detail calculations (aggregate method) and be in compliance with Federal and State tax withholding requirements.
Hope this helps.