When is a Cardboard Box Worth $18.10

I mentioned in an earlier post about needing repair on my magnifying lamp.  When the repair was completed (with six replacement parts), I received a bill for: $20.25 in parts, $36.00 in labor, $20.29 for return shipping, and - get this - $18.10 for the shipping container!  I have never before had a company charge me for the cardboard box! Especially since I sent the lamp to them in a cardboard box that they could have re-used had they wanted to....  I can't figure out why they don't add a penny or two to each of their parts, to cover the cost of the cardboard boxes.  That would be a lot easier to tolerate than having to pay for the box.