A "stop charge" would be the amount of fixed cost assigned to the service. Then you would add a proportional charge for the goods supplied and the time to effect the service.
Grossly simplified example: you have determined that, over the year, each time you stop for an application the cost (from fixed business costs) is $11.87. You apply 40# of product that you will charge the customer $18.50 for, and you spend 15 minutes at $100/hr (=$25). So your charge for the stop is $ 55.37.
If you are a big business, and just work on averages, you can charge $XX per thousand. On some you will lose money, on most you will make money, and on a few you will make a killing. If you prefer to know that each job is profitable, then you need to work with the numbers, or hire someone to do it for you.