For an off-track call we carry replacement rollers in both nylon and steel, sections of track in the common widths, lift cables sized for single and double doors, and the drums, bearing plates, and end brackets that usually take the load when a door jumps its rail. We also carry torsion springs and stabilizer bars, because a door that's off track from a broken spring needs that fixed first - the track is rarely the root problem, it's what gave way after something else did.
What we don't carry on the truck is a matching replacement panel for every door style out there. If a section is bent badly enough that straightening it won't hold, and the manufacturer no longer makes a matching panel, we'll tell you that on the spot rather than order a mismatched piece and charge you for two trips. Sometimes the honest fix at that point is a new door, not a patched one.