$7,000 of credits is no problem. At that time a friendly neighborhood PM or director could issue the credit without much oversight.
Your problem is the time period. Amending a bill in the same cycle is EZ. Fixing the previous cycle is a PITA but pretty common. Issuing amendments for the previous financial _years_ would be a huuuuge PITA going through finance etc.
If you're not watching your billing, and then try to claim overcharging a year later, you'll get a lot less satisfaction even from regulators or judges than if you notice it when (or soon after) it happens.
The employees and company have an obligation not to exploit this even if the issue is only discovered after the fact.
You don't get to export any of the responsibility to your customer. They don't prepare the bill and it's not their job to find your fuck ups
I once got a monthly water bill for ~$35,000 at a residential, single-family home. Good thing I was paying attention and looked at the bill before the auto-pay bank draft hit.
Someone had misread the meter.