Worst client service ever

Employees are not properly trained to assist customers with medical aid. I bought a Blood Pressure monitor at Clicks. When paying I asked the 2 ladies at the Cashier if they can give me an Invoice for the medical aid. They said the till slip will be sufficient. Trying to claim from the medical aid they advised that indeed they need a Invoice. Upon enquiring for an Invoice they advised that it is not possible as I had to go through the dispensary. Why Clicks, after your staff made a mistake? I still have the original till slip as well??