Discovery Zone Children's Center

This business failed to properly bill me correctly for my child care for three months, and was not charging for me 154.00 from January through March with and error in there own system. When they found the error they are now trying to charge the past pay that they failed to bill me. I do not feel it is right that since every month I paid the balance due and it showed a 0.00 balance and now they are charging an additional 300$ for supposed back pay.