Print out the invoice, and then send it with a check for $24 and letter requiring them process the check against invoice to their legal department via certified USPS letter.
Ah yes, Microsoft, bringing us back to the 1930's methods of financial transactions.
It is worth a try. I have had really good luck with the US Postal Service. People do not get much mail, so if you send it, someone might actually see it. That will never happen with email or any electronic tools.