Sorry to hear about that.
Like you said, all your costs are visible and up-front, so who can complain? Sadly, we have bred a society where a sizeable chunk of it thinks it deserves everything and has to earn nothing. This is probably reflected in your ratings with people wondering how you have the nerve to ask for a penny more than the actual shipping price on the box, with no thought about time & effort, the cost of the box, the cost of the packing, etc.