Pay back the money!!

my mother paid cash for invoice # ********** 6. the package inside was not what she ordered. she sent the package back and parcel number is PA ********** 57ZA...she advised one of the consultants there that she would like to be refunded her money she paid after she was rudely advised that the parcel had only 2 blankest instead of 3..now tell me, I have both slips here with me on the invoice it says each weighs 4kg x3 which is 12kg...when she returned the goods on post office the stamp says 12kg as well. why my mother was harassed when she asked for a refund cos you sent the wrong goods in the first place and she had to pay to return them to you? I need that money paid back to her, she doesn't want replacements.