Debtors Error

I contracted COVID in August and was admitted to ICU for 4 weeks. During this time I underwent many bloodtests from numerous laboratories. This was undertaken by the attending doctor and I had no knowledge of any specifics. on 23 December I received a call from Vermaak asking for payment for bloodtests. First I heard of it as all payments had been done months before. This was the first contact from the lab I have had and I gave them my email address for them to send me an invoice so I could take it up with my Medical Aid. I contacted my medical aid (this is covered by my hospital plan) and they undertook to resolve it in the next few days. I copied Vermaak in on this exchange. To my surprise, this morning Vermaak sent me a statement and final demand letter threatening legal action if not paid within 20 days. So basically I have still received no Invoice, no detail of the actual procedure, only been made aware of the debt for one day and this Vermaak is threatening legal action? All this for a debt of under R300? Very poor show.