The Union Contract requires that for every seven regular drivers hired, six must be by seniority from the existing UPS part-timer ranks. The seventh can be hired from the outside, but any part-time supervisors who want to go full-time driving get first shot at that seventh slot.
As a temporary driver you probably don't accrue seniority, and would have to give up your driving job when the temporary period is over. At that point you may be offered regular part-time (3.5 hours per day) work at $8.50 to $10.00 per hour or so, or you may be laid off. If kept on as a part-timer, it could take several years before you could suscessfully bid into package car driving.