You got paid by the hour...you get overtime.
That's your bonus.
Being a part time supervisor is absolutely the most thankless job in the company. It's honestly a job meant to get you through college and pad your resume.
Two points.
1 - Don't come to the Brown Cafe' and ask questions about being a management person. As you can see from the responses, there is a core group of current/retired teamsters that post on this website that will mock your questions and/or comments. Ask your boss how you're doing and what it will take to move up in the organization.
2 - The part time supervisor position was a great thing for UPS and for the successful part time supervisor. If you speak to your boss and your results put you in the "middle of the pack" as compared to your peers, then finish your degree and as OrionisDaddy said "pad your resume'" and find another job.
If however, you are doing well and there is a path to full time management open to you- take it. I worked in a hub once where every full-time management person, from the supervisors, managers to the division manager had all been part-time supervisors at the start of their careers.
I worked in a district where the district manager /president had started their management career as a part-time supervisor and if you look at UPSers.com, you'll see that many district and region managers worked as part-time supervisors at the start of their careers.
Your job is not easy, but it is important.
Talk to your boss. See how you're doing and make an informed decision. Good luck!