Commissions are the established way for sales

A simple example of a commission scheme…

Management sets quota for sales, say 1000 widgets in 2013

If the salesperson sells less than quota, they get the base salary

Salespeople who get more than their quota gain a substantial commission, in addition to their base salary.

There are many kinds of commission scheme, but the common feature is tying a salesperson's income to the sales they made. If they don't make their quota, they don't get their OTE (On Target Earnings).

Commissions are popular since they align incentives. This way the salesperson earns money depends on their sales - which is what the employer wants.