Many people today have given up on justice prevailing in hearings, on invisible union representation, or they are just using this (part time) job to fill in until they find full time employment elsewhere.
"The union" means little to them at $11/hr.
The work atmosphere is often caustic and the carrot to make them stay for the first year looks pretty scrawny.
Get the part time starting wage up to $15+/hr and give them benefits after 90 days then "the union" may mean something to the average worker.
Otherwise, it's an abusive job with little promise and "the union" may look self serving with the initiation fees and dues wiping out a big chunk of their meager earnings for a few weeks.
Looks to me like the union officers need to step up their game and show up on the shop floors these days. Right to Work looks pretty tempting to many part time UPS employees.