Three weeks, numerous phone calls, two incorrect/incomplete invoices, no solution, no promised contact from a specialist

I had to wade through the website and automated voice-mail maze to even get close to people who told me they *not me* could directly contact someone who could help, a "specialist", but so far no contact with any specialist, no resolution to my problem after three weeks of trying.

And now I get "You have replied to a case that has already been closed" when I tried to reply to an email I received stating a specialist would contact me.

What. do. I. do, now?? What is my next step? When can I talk directly to a human with the ability to solve the issue?