The use of such statements from the un-knowledgeable is a difficult one to deal with as we are all consumers and we all make purchases throughout our days weather a clothing purchase at a local store or a meal out, so the feeling of getting a deal or a good bank for the buck is in the eyes of that consumer.
Now should you have cost shifted by inflating one area higher or charge for something not needed "The impeded hoadie valve removal of built up carbon" perhaps would have been easier for them to understand then a professional working on my car utilizing the most updated modern equipment available to get the diagnosis right we need to be paid for this service needed and necessary, fairly honestly and ethically.
A local shop here offers the free use of any and all of their scan tools and folks actually pull in and play with them and I guess it works for selling work, and then the big independent tire store offers free diagnostics now they need to educate the staff on just how to use the darn stuff as not every car needs a forward O2 and Cat.
Oh well a lingering topic for most shops, but nonetheless a worthy one to start the day with!