It might be a genuine mistake, but is still the responsibility of the store to make things right. While the OP might be a little strong here, who sends an INVOICE and has the incorrect TOTAL? That to me is absurd and sloppy and if it were my business I would try to work with the buyer as opposed to asking them to understand their point of view.
The buyer shouldnt be fixing the sellers sloppy business practices, just my point of view