Harassed for outstanding amount but refuses to provide an invoice

I am writing this on Hello Peter because trying to get proper feedback from this company directly has been a nightmare and I now feel forced into paying over R1000 on a bill that I am apparently outstanding despite having never heard of the company or received a single invoice for fear of being handed over.

All I have asked for is an invoice so that I can see what I owe this money for, and I will then gladly make payment. Surely in today's day and age, we have a right to an invoice for services rendered before we make payment, especially since I don't even know what this bill is for.

I am appalled at the way I have been treated by 'ER Consulting' and their debt collecting agents. A company I have never even heard of before suddenly been harrassed by them for some apparent outstanding debt that I have. After repeatedly asking for an invoice to no avail, I was promised a statement within no later than Thursday last week (16 Jan 2020).

Today (20 Jan 2020) I suddenly get a letter of demand on email threatening to hand me over if I don't make urgent payment, including an easy way to pay directly to them. It also includes how I could be charged extra for asking for documentation of them since it is apparently my duty to figure out what I owe this money for on my own. However still no invoice. Would it not be advisable to attache the proof of services rendered, and an invoice to said letter of demand?

I actually can't believe that it could be so difficult to provide me with an invoice if you are expecting payment. How can you go around handing people over without ever communicating with them in the first place by sending an invoice for services rendered, and then treating them like ********* for simply asking what they owe the money for? It is a simple business practice to send out invoices but this is apparently beyond you.