Rounding every parcel to a whole cent
This is the one nobody writes about. A parcel’s fee is a fraction of a cent, and because every parcel of a store shares one packaging composition, rounding each one loses the same fraction every time. It is not noise that cancels out over a month; it is a constant, one-directional error.
On the example figures it is over a hundred euros a year, on a cost of about a thousand. The fix is to accumulate in thousandths of a cent and round once, which is what the product does.
