Building interest on account to charge customers more without informing them they have an account.

Disgusting. Please note this is the second time this has happened. I will use their services, they submit to medical aid and contact me if it didn't pay. 6 MONTHS later the bill surged because they were charging interest and only informed me of the account with no contact this week. I told them that if they needed to be payed, then just inform me. Ultimately I payed the full amount for the services however REFUSED to pay interest for something brought to my attention after several months of interest accrued. Now they want to list me for not paying the interest. Disgusting (happened last time aswell)