I know I've come to be known somewhat as a sub basher, but I actually am a basher of any worker who doesn't do their job. If there is a carrier, sub or regular, that is leaving work for someone else to do for them it shouldn't be tolerated. Never is it proper for anyone to leave work for another carrier unless there are some circumstances that prevented the normal delivery of the mail, weather, fire etc. Even when these circumstances occur it doesn't mean the next carrier has to do their days work and the work of the previous carrier, without additional pay. If some boxes weren't able to be delivered due to weather then it is expected that the next carrier would deliver the extra load the next day, but if there is a large # of deliveries that need to be delivered the next day the carrier should be paid extra or given auxiliary help to do that extra work. However, work that is left because the carrier was "behind/had a bad day etc." should not be tolerated. The way that any carrier, that is having trouble with work being left by the previous carrier, stops that from being done, is to ask for pay for the extra time you take doing that extra work. You only have a grievance after management refuses to pay or give aux help to cover the time it takes you to do the extra work, when this happens file away.
I explain it this way to my managers, "We are piece work people (the pieces for one day's work) and when we get more pieces (portions of another day) we get more money. You wouldn't think of going to your mechanic's shop asking for a brake job and coming back and asking why didn't you do the shocks also for the same price? That would be ridiculous, as it is ridiculous for you to expect me to do the work you allowed to be left for me with no extra compensation". If you talk slowly enough, often even supervisors can come to understand this, but if they don't, they will after you win your grievance.