Overhead. Suppose you spent $12,000 for overhead last year.
Suppose you have 250 customers.
Suppose you do each customer 5 times, that would be 1250 applications.
Divide $12,000 by 1250. You must charge each customer $9.60 per application for overhead.
That overhead fee should be part of the stop charge, (base fee). Then you need a few dollars more to cover the average labor time and vehicle expense to arrive at the location.
I know of a big company that used to have a $22 stop charge.