@Mart Man
$500 is kind of steep in my opinion for a temporary seasonal worker that is allow to work for 3 months.
$500 / 3 = $166.66 init fee per month
The problem is with the UPS payroll. If they charged you $166.66 on your first week, you end up with (approximately $0 on the first week) provided that you had a part time job. We didn't see your paystub so there is no telling what is the dollar amount and how many hours work that you work. This is not to says that I am blaming you. $500 is still too high.
When the management told you to blame the Steward, they basically threw the stewards under the bus. Steward had no control or access to your payroll nor the union's init fee and union dues. Their job is to make sure that your contract is follow and assist in verifying your position in the union is in order.
The UPS seasonal job is a pay to win. UPS is offering seasonal bonus provided that you attend all the day to earn the bonus money. UPS isn't for everyone, and people quit. People's quitting is UPS's gains as they save money by not paying medical insurance while conscripting the workforce to meets the high volume demand. In other view, if you did survive the UPS seasonal and apply for perm, UPS may or may not hire because you have month credited toward your medical insurance eligibility. On the other hand, if you work as a permanent position, the bonus is really a hiring bonus. Should you apply as a permanent position instead of seasonal, you wouldn't have earn the bonus in some situation. I know there is a on-going bonus for permanent member because of the contract proposal, but I digress.
The bottom line is that you and other got shank. UPS used the union as a weapon. If it was deliberate, they use it to get rid of you and other for free labors to avoid paying out seasonal bonus.
I am sorry for your loss and hope that contacting the Payroll in collaboration with the union can help get resolve. I fear that the damage is already done and that they cannot "reverse" the damage unless there is a major negative PR involves.