Invoice lost on 4 occasions.

I had submitted an invoice for a medical account which I had settled last year November 2015 and did the necessary to make sure that this was emailed in a timely manner to be settled. I followed up early Jan 2016 but was told that the invoice is not reflecting on their system. I once again sent the invoice but made sure I got a response stating they had received it - again it was not scanned and not on their system - I followed up again and guess what - same story - today I followed up again and surprise surprise - no invoice on system. This would be the 4th time - be careful of this medical aid company doing all it can to avoid making payments.