The issue with Mack started years ago. And UPS does not forget. One of the reasons they started ordering Kenworths and Freightliners. It used to be just Macks and Internationals. There were Fords (Sterlings), but only for a few years.
The deal with Mack was that UPS required springs, not air bags on the rear axle. It all boiled down to cost. The springs were cheaper to maintain than air bags.
Macks started coming standard with air bags. So Mack would have to build all their trucks for everyone else, and then at the end of that model year, would have to retool the assembly line to build the tractors for UPS with springs.
Mack finally got tired of doing this, and told UPS to take or leave it over the air bags.
I believe the 273 series Macks were the first ones we got with the air bags. But this was the start of negotiations with Kenworth and Freightliner. I heard there might be some Petes coming in also. They got away from Macks for a long time, but have started ordering some again.