I finally broke down and bought a generic USB-C charger (Insignia from Best Buy) to keep in the shop, because enough USB-C laptops come through and nobody ever brings a charger.

But just because a laptop has USB-C ports doesn't mean you can charge using it, here's looking at you, Lenovo P51. This machine is a workstation and requires 130W, USB-C can only supply 100W so it won't charge at all over USB-C. Blah.

Also had no luck even trickle-charging smaller USB-C machines like the HP x360 using a phone charger.