You'd have to start as an entry level mail clerk. Rather than rely on looking at job postings, prepare resumes and cover letters and approach the hiring departments of companies who hire mail clerks specifying you are looking for an entry level mail room position. Sell your skills as being motivated to learn, able to be on your feet most of the day etc.
You can also approach companies for "informational interviews" which means in your case asking a few questions about how you can get hired as an entry level mail clerk -- what are the qualifications? who does the hiring and when? Again, always sell your skills for the position and be polite while being persistent.