Billed 2 Years After Service

I am extremely irritated with Vumatel. In 2017 I had fibre installed at my phone, of which I was under the impression that my ISP (Cool Ideas) covered all the initial costs. Turns out, there was an installation fee from Vumatel for the actual labour, of which Vumatel careless forgot to send me the invoice for. And now, almost 2 years later, I am sent a message that I have an unpaid invoice. Their explanation is that they were just now doing their books and realised there were accounts they didnt invoice. TWO YEARS. How did this affect my credit score? and further more, this was money I had not budgeted for which has just f*ked up my budget. Really *********** and unprofessional.