Unwarranted, threatening emails from this company to pay a bill

I went for an annual check up with my doctor and paid my invoice only to receive an invoice from Lancet demanding a payment for under R300 with a threatening warning that I was a month overdue. I never knew about the invoice and always pay my bills on time, so I was surprised by their rude email and their aggressiveness. So i emailed them for bank details, as I was unable to open my e-statement because there link doesnt work, and I received details for a bank account. My bank then rejected the details as the account number was invalid.

This company is rude, and has wasted so much of my time already, I hope to never use their service again.