Just stop coming. Your supervisor is just taking advantage of your kindness/naivety. He's not your friend. He wants you to stay because you show up and try... which is more than 99% of new hires do. He'll string you along as long as you let him. If you want to quit, just quit. You aren't ruining your "career" and no one is ever going to care.
I'd say about 10% of new hires quit after a day or two, 25% of the remaining new hires quit after they get their first check (and I don't blame them, starting pay is way out of line with the amount of work expected of you). 50% of the remaining new hires quit before their probation is over, and 99% of those remaining will quit before their first year is up. It's a terrible job in a terrible environment with very little payoff, especially in the beginning.
There are only a few reasons to work for UPS... you put up with the BS if you 1) want to become a driver, 2) want tuition reimbursement, or most commonly 3) have a family and want the insurance benefits. Most of the people I work with (including myself) are here for the benefits. If you aren't looking for any of these three things, just get out now. It's not worth it. No one (especially anyone who has ever worked for UPS) is ever going to blame you for quitting. Just stop showing up, now, or after your two weeks, and put the nightmare behind you. It's not even worth worrying about.