If they are doing business I am sure there is a contractual obligation for Amazon to reply to any invoice issued with payment or reason for non-payment. That being said, enforcing the contract and going to court is a last resort, as you're probably not going to get a renewal when the contract ends. I want to shut my client down right now for unpaid invoices. It would make my job so much easier, but then I would probably be out of a job in 12 months. All the time I see invoices that are past due, I send it out to the person to arrange payment, and get ignored. 3 months later the reply is: "send it to person xxxx" send it to that person, get ignorted for 3 months. that person quits, send it to their boss, get ignored. Then, when you get a reply it's something like "this invoice is a year old, it shoud have been paid out of last years budget, I think you should write it off. why didn't you send this earlier?"
I feel for this company. Best way to stay rich is to not pay your bills.