Answer:
I would use progressive commission sales, for example:
number of units sold sales commission
0 - 200 2%
201 - 400 2.5%
401 - 600 3%
601 - 800 3.5%
801 - 1,000 4%
This way, salespeople will be motivated to sell more units, since they know that their income will increase dramatically as there sales volume increases.