You, contractually, owe UPS an honest day's work for an honest day's pay. There is no language in the contract providing for any 'pieces per hour' minimums or any other such nonsense. Do the best work you are capable of, work as quickly and safely as you can. DOCUMENT DATES, TIMES AND WORDS, Of any supervisors who demand more. Contractually, you are also entitled to be treated with dignity and respect.
The first step is to say to your sup you are working as safely and quickly as is reasonable, and request that he/she be dignified and respectful in their speach. Failing that, bring it to your steward, and ask others nearby to make witness statements to the steward.