This concession on UPS' part wasn't even a concession. It's like going to a dealership now and the salesperson offers you a free upgrade for automatic windows over the hand crank windows.
It costs less to get the A/C feature than to take it out....just like power steering feature.
UPS complained publicly about the frequent stop, doors opening, and trucks being off issue that wasn't really an engineering problem at all. It was a UPS didn't want to spend a nickel issue. See no-idle air conditioners with high capacity cooling exist. They are battery operated. You've probably seen them on top of campers and RV's. They can be easily retrofitted. It just costs more than nothing. But it is more than gluing a piece of aluminum foil around the frame and installing a fan so you know what the company wants to spend until it costs them nothing for the option on a new truck.
Want to let the company go the cheap way and have basically an under-powered A/C that is no better than a fan with a cup of ice in front of it--let UPS work out the details. Want to have a cool cabin: put the words no-idle air conditioners with high cooling capacity in the contract.