We have a CCA in our office that is 63 years old and has a bum leg, and all of our supervisors don't care how slow he is, they work with him and allow him enough time to do the job - they'll maybe give him half a route because they know he'll take twice as long to deliver it, or give him less parcels to deliver on a run, those sort of things.
Unfortunately they can fire you for any reason during your probation period. However, in my office a CCA was about to be fired last month after she drove an LLV into a parking garage that didn't have enough clearance. She ended up tearing the roof open. But because she was a really good worker, the union steward stepped in and saved her job. So who knows? If you do get fired, talk to your steward and see what they have to say.
I'm 3 years into this and just converted from CCA to PTF last month. It's nice to have a retirement and savings plan, a solid health plan and affordable dental and vision, along with a free life insurance policy. The paychecks are less than what I was making as a CCA because of all these new deductions, so it is a paycut, but the job does become more solid as you learn the routes in your office, find your rhythm and have an actual schedule with two days off a week.