I charge 8.01% on total ticket with a cap of $39.97. I have been doing this for over 1-1/2 and never had anyone complain about it. I have a list plainly displayed on the wall of what it covers. Anything that you would use more than once (brake cleaner, grease, ect) is what is covered under the shop supplies. Anything that is a one time thing (clamps, bulbs, ect) I charge on the ticket itself as a part.