How is charging by weight discriminatory or unethical? Assuming the same rate per kg for all comers, of course.
Having said that, imagine she bun fight at the airport when Bloggs and/or Bloggsette roll up having booked and paid on the interweb, and the airport scales who that they've either not been entirely truthful, or, the home scales just happen to under-read. And so they fight the demand for the extra dollars.