Word Choice

The sentence is verygood, I would give it a couple of tweaks.

I take it you're the party who hasn't paid invoices, in which case so seems a bit harsh, cause and effect etc.

I would change so to I'm afraid which sounds more culpable and friendly.

Also put a comma after open.

That's it.

The email address you have is correct. I will investigate the open invoices issue and report back as soon as possible.