incorrectly billed over and over again

i recently wrote a report about the incorrect bill from foshini store, it is so frastrating as not one seems to give answeres my mom has an account with foshini and she has a credit limit of R650.00 she only used the account once in 2013 when she bout a watch at stens, she is always paying her account not the account is standing at R1700. which i dont understand as she has a credit limit of R650.00, what she needs to find out is why the account is sitting at the amount as she does not have a buying power at all, this is a **************** no one seems to explain why this is happening