Every one wants a deal. You have a business to run and deserve to make a living. I will assume that your rates are posted both in your shop and on the trucks? If you are towing to a body shop what is the issue? The shop pays and bills the carrier.
In your market place are you priced as fair?
Customers have asked me why I charge what I charge and want to chip it down. Response, go find someone else who does what I do and use them. Ha ha, there is no one else who will fight the insurance carrier to get a fair settlement on totals etc.
My business continues to grow and I do diminished value on some pretty high stuff like lambos, porsche and RR. When people see the end result they recommend me.
All the responses pretty much tell the same story, you have a business to run and deserve to make a living. Those who complain should find another source.