The serpentine belt is part of a system and is not the only component susceptible to wear. The tensioner on an engine wear over time and need periodic inspection and replacement. If you are replacing just the belt without looking at the other components, you could be sending a new belt or an even more expensive component to a premature death. Sell the complete job up front. Include the tensioner on the estimate for the customer. The complete job will make the belt last longer without having the customer comeback to finish the job piecemeal.
This video is sponsored by Continental.