Help please read below , I’m trying to shipped

Please help !! I’m trying to send my item that I sold it’s a backpack so I need a bigger box , they won’t accept it because is supposedly my box is too big they want me to buy a label for 13 pounds when this box weights only 4 lbs they want me to pay $50 I don’t get it ? , they said the bigges I can go with this label is 12 by 12 by 12 and this box is 18 by 16 by 7help please thanks I know they can do it I have received bigger boxes from here when I bought stuff