In the parent org account go to https://us-east-1.console.aws.amazon.com/organizations/v2/ho... and you can delete the child account and you should just be on the hook for the 90 days closure period. I think you can also just disassociate (revoke the invitation or similar) the account from the same place.
Unfortunately, the guy who actually ran this account fat fingered one of the addresses (name@abc.cm instead of name@abc.com) - so there is no way to access the account, put in a credit card, and disassociate it.
I asked them what the process would be if myself or one of these former employees had died - and was given the run-around. Every single thing they told me to do did not work and just exposed another exception.
There are multiple contacts in amazon -- Billing, Operations, and Security. If you have access to the Billing email you should use that for contacting them.
Otherwise it looks like a credit card dispute might trigger a cancellation or similar but I don't know for sure.
I asked them what the process would be if myself or one of these former employees had died - and was given the run-around. Every single thing they told me to do did not work and just exposed another exception.