Would you (Kodie, Sam) consider hiring a third mechanic since the current two are so busy?
P-buses actually save wear and tear on Arrow buses. P-buses are used on long trips. If the P-buses just disappeared, you'd be stuck working on the route buses more often, anyway.
This in mind, how about a compromise. P-bus owners pay for materials and labor for upgrades, materials for breakdowns. For routine maintenance, no fee to the owner.