The fitness instructor invoice can be used by any fitness professional who gives group or individual fitness instruction. Fitness instruction usually refers to group fitness such as aerobics, Zumba, aquatic fitness, spin classes, or CrossFit. However, it can also refer to the instruction of various sports and other forms of exercise. Instructors can charge their clients a fixed amount per class, a regular subscription (weekly, monthly, etc.), or an hourly rate ($/hr).