I would recommend to check customer support, and maybe raise a ticket...
This is a bit peculiar , even if you have access to Order History then N.I. apparently does not allow for print or download of the 'original invoices'. The standard N.I. help page "I Need a Copy of My Invoice" is here (and same for Izotope is here) , but that apparently does not help since you already have tried contacting support more times.
So this is definitely a case for @Jeremy_NI to take a look at ! (by 'tagging' Jeremy I just called for him and he should hopefully read this on workdays Monday or Tuesday next week)
I can't do much on that one, I don't have access to all the data. Your initial request is still in there so I pinged my colleagues. They should get back to you as soon as possible. The second ticket was only "set to solved" because there was already a request for the same issue.