I've worked in a few mail centres. I usually is aggregate hours, with a reset at the start of each financial year.
I've seen systems where part-timers get pro-rata credit for the overtime hours they work. Thus, if you're on something like a twenty-hour contract, you get eight hours credit for working a four-hour overtime shift.
This is meant to keep the part-timers from getting all the overtime. In practice, though, since they have more rest days, and/or are only working four-hour duty shifts, they do anyway.