I would offer a reasonable base rate with minimal deliverables and then bonuses/profiting sharing for anything above and beyond that. Talk with your accountant or bookkeeper to figure out what works best.
I've seen 12% to 18% with a sliding downward scale yearly. Some Co.'s have a "draw" to pull from each week if things are slow. One thing I always hear of is massive bonus's if goals are met or exceeded. I know allot of sales rep's in this area who are making a fortune off Winter Moth treatments.