According to Insert 101 of the Summary Plan Description booklet for the UPS National Health Plan for Part-Time Employees:
"Once you have gained eligibility, you must work a minimum of 40 hours each month to continue your eligibility for coverage under the Plan. Hours worked in one month qualify you for coverage two months later."
Any hours worked over 50 hours in a calendar month are banked. The maximum number of hours that can be banked are 240 hours. If you are short of the 40 hours in a calendar month, hours will be taken from the banked hours in order to meet the 40-hour minimum requirement. If there are not enough hours in the bank to meet the 40-hour minimum requirement, then your coverage ends the first day of the second month. If you lose coverage for a month, you may elect to continue coverage through COBRA in accordance with the Summary Plan Description booklet.
Hope that clears it up. Theoretically, all part-timers are covered under the same national UPS health plan for part-timers.