How the cost is worked out
Parcels returned each month = parcels shipped × the share that comes back. Each return costs the courier charge, plus the return charge, plus packing. Multiply the two and you have what returns cost you a month.
That is the money you can count. Returns cost more than that: stock sits in a courier's van for days instead of on sale, some comes back damaged, and the cash you expected doesn't arrive. The calculator leaves those out, so the real figure is higher.