Your time is the largest invisible line
Setup is paid per plate: clearing the bed, starting the job, checking the first layer. Post-processing is paid per part, because supports come off each one. Counting both as free is what keeps a print business looking profitable while the bank account says otherwise.
Setup divides, cleanup does not
Eight parts on one plate share a single setup; each of them still needs its own supports removed and its own edges sanded. That asymmetry is the whole reason a fuller plate is cheaper per part — and the reason a larger order is not automatically cheaper per part.
The small parts that never reach the spreadsheet
Magnets, screws, threaded inserts, a bag, a box, the label. Individually trivial, together often more than the plastic — and unlike the plastic, they are paid for in cash at the moment you ship.