Is our invoicing correct?

Our company is not VAT registered. Our company has been issuing invoices to clients. The invoices clearly indicate that we are a non-vendor and we don't charge VAT. Though our invoice says tax invoice. Is this a problem? I am going for a VAT interview soon.

They should not say tax invoice on them, but the rest sounds correct.

So what you are saying is that we should correct them to say Invoice instead.

Yes, it needs to only say Invoice and not Tax Invoice. As silly as it sounds, SARS only allows VAT invoices to have the words Tax Invoice.