While there may be differences in what you do each day compared to the mail room clerks, the types of responsibilities, types and levels of skills required to do the work, the knowledge and/or accountability involved and so forth may be so similar in nature that the manager felt that it just made good sense to eliminate a job title that was no longer justified.
Without more information about the differences as well as the similarities of the UPS clerk versus the mail room clerk job, it is difficult to speculate.
An employer may hire someone for a particular job and then the job is eliminated or changed over time. Just because the employer hired someone into a specific job does not mean that the employer is obligated to maintain that specific job no matter what transpires regarding technology, for example, and other factors that affect company operations.
The employee always has the right to leave if the changes in the job title, job requirements or responsibilities are not acceptable to him/her. But employers have the right to determine what workforce they need to run the company efficiently and effectively--and profitably. In fact, that is one of their principal responsibilities.