I wish they had opted for CNG instead of propane. Propane is useful for too many other things that methane is not, like home heating and chemical feedstock.
They are using CNG as well. They haven’t picked a single alternative fuel source, at least not yet.
UPS has one of the largest private fleets of CNG vehicles in the U.S., with more than 965 package delivery vehicles. UPS began extensively using CNG in 1989 to assess its benefits and viability as an alternative fuel.