Terrible service and feedback/communication.

I have a medical practice and rented an O2 cylinder from afrox as a back-up during the Covid-19 period as my personal cylinder was faulty. After 2 or 3 months my cylinder was replaced and I returned a totally unused, full cylinder to them. Unfortunately one of their employees has removed the seal on the day I got it, without my permission, to check if my regulator fits the bottle. If he knew his job he would have been able to look at the regulator and say that it is the correct one for the bottle. Because of this I forfeited any possible debit or discount. The next mont I received an invoice for the bottle again. I spoke with the accounts department and they said that they would rectify the error. However, that was not done and I still receive invoices for the extra month charged. Eventually after a long struggle the accounts department informed me that the extra month was charged because I returned the cilinder 3 days after their month-end which is around the 20th. Never ever did anyone explain this to me and that I will be billed for a whole month should I return the cilinder after month end. In other words: I got billed for a full 28 days, during which I did not even have the thing, and this for essential medical products during a covid pandemic. Not to even mention that it was still full. I informed them that I would pay for the 3 days I had the cylinder, and I did so, but that I am not prepared to pay the rest. Up to now I have had no feedback on my response and continue to receive the invoice every month. What terrible service, I will never ever again refer a patient to them for home oxygen!!!!