The union is fighting about UPS PT wages so that it can go to Amazon's PT workers and say see look what becoming a union member can do for you. It's all about dues and membership ranks. Without that growth the union knows its long-term future will be an ever decreasing rank and file as automation claims more and more jobs.
Obviously, they can't do that when Amazon's PT workers are making more than UPS PT workers. Can talk about benefits till your blue in the face but turnover ensures a large majority don't even hit that mark. And talking about working conditions only goes so far.
The union knows that they have the leverage in this negotiation thanks to two things:
1) a 3.6% unemployment rate
2) a low labor force participation rate (boomers retiring, slower population growth, etc)
The unemployment rates in 25 states are currently at or within 0.1 percentage point of a record low and there is less of a pool to draw from. It's even lower when taking into account low wage workers as their is lower interest in low paying jobs because of inflation. Meaning there is market pressure on UPS to raise rates aside from the union's bargaining pressure.
So you can talk about too high PT increases all you want...but the union is not going to be put in a position where the company is giving PT'ers more than the contract demands because they can't find anybody and then they give bonuses to area A and not to area B. That's right out of the how to break a union playbook.