The law is more about what questions are not allowed to be asked than simply what to ask.
But knowing what not to ask has created a general guideline of what to ask for an employer.
Unless your job involves direct contact with your customer than your answers shouldn't matter outside of the ones pertaining to your legal working status and how capable you are of completing the job.
Most PPT employees start out loading trucks. Not easy at first but you figure it out pretty quickly, little thought.
No thought goes into unloading.
Bagging small sorts is just as mind numbing.
Sorting small, pick off, and working the sort aisle are the only three positions on the typical evening/night shift (twilight/reload, whatever you prefer) that require any sort of brain function and you receive an extra $1 at many centers for being less than a drone.
TL;DR
My point to the question asked is: your answer isn't that important, they asked because its what they do. Tell them you want to work and that you're always available (reliable transportation, can work shifts where the start time isn't the same on a daily or weekly basis and you finish when you finish, no set time) and your name skyrockets to the top of the list essentially.