Is it easy to move up to a better position if you start as a USPS casual clerk?

My husband took the USPS test and now has ben hired as a USPS casual clerk. The thing is, accepting this job will be a big move for us...we will need to move an hour and a half away...not something I want to do unless it provides the potential to later move in to a higher paying position, like rural carrier or another job within the USPS. So basically, is it easier to get yourself a good job within the USPS if you start at the bottom with this job? This is not what we want to be a lifetime job for him.