Full time employees were salaried, with 8h work day averaged over a month. When work was done, you went home. If you worked 6h one day, 10h the next you would not get overtime. Overall, I would guess I worked about 80% of the monthly hours, and I never hit overtime. Longest day I worked was 13h, shortest day was 3.5h.
Temps were hourly.
The extra time cost of adding a piece of mail to a route is mostly negligible. There are a few exceptions.
The first piece of mail that will take you inside an apartment complex adds a good bit of time (since you have to get off your bike and go into the building). I may have occasionally have forgotten to deliver something that was going to the 4th floor if that was the only thing in that apartment complex on a given day, and delivered it the next day instead.
The first piece of mail to a house doesn't matter - 95% of the time delivering to houses is the transport time between houses, not the delivery.
For us, routes were typically limited by physical effort and in-between transportation time, not the volume.
Houses were limited by the time of going from house to house, to about 2.5h time total. Apartments were limited by the effort of going up and down stairs to about the same time.
A usual route would be ~600 houses, or ~80 4-story apartment entrances. High rises were more limited by volume (and were typically mixed with 4-stories for that reason) so that a normal route would be maybe 8 high rises and 70ish apartment entrances.