Why must I pay for your mistakes?

I had an outstanding gynae bill from March of which incurred interest. I phoned Bonitas and they said I would need a referral letter for the gynae (bearing in mind I have been to this gynae for 2 years and have disclosed I have PCOS which means I need gynae care.) Anyway the call centre agent admitted it was a glitch on their side because they could see that there is a not of a pre existing condition. I then asked who is liable for the interest as it isn't my fault the gynae wasn't paid? The call centre agent said she would log a query and my husband would receive all the emails detailing the situation. My husband received an email stating they will or have paid the original bill but what about the interest???? Thats bad service and we pay so much every month, for what if you make such silly mistakes that could have gotten my husband blacklisted or handed over.