Answer: Straight piecework plan
Step-by-step explanation:
The Straight piecework plan is a type of incentive system in which the rate per unit output is constant and fixed at a specific value. And the total earnings of a worker is calculated by multiplying the value of its total output by the rate per unit output.
In the case above the rate per unit invoice processed is fixed, so employees are paid based on the number of invoice processed per hour which makes it a Straight piecework plan system.