Typically sales representatives collect earnings based on a percentage of sales. The percentage varies and that is something you will negotiate with your client but consider that commission can be as low as 2%, as high as 20% and most commonly falls between 5% and 10%.
A contract should be drawn up detailing the commission rate and the length of the work period. Usually contracts also include a guarantee of payment for an extended period of time, sometimes 6 months, beyond the original contract date. This ensures that any late sales that result from the marketing campaign you produced are also credited to you at the same commission rate.