Up until 3-4 years ago, if you created an AWS account with the same email address as your amazon.com shopping account, Amazon would automatically link the two together, without warning and without a setting to undo this. Changing the password on one changed the other too. This connection persisted for accounts made after AWS stopped doing this, probably for backwards compatibility reasons.
At least when I dealt with this mess a couple years ago, AWS support did not have reassuring answers. They claimed they could separate it, but if something went wrong we were on our own without spending large amounts of money on premium support.
Lucky for me most of our resources were already in child AWS accounts and the rest could be migrated, so I was able to create a _new_ top-level AWS account, re-parent the child accounts, and delete the old parent account instead. At least with that process we could create a throwaway child account and test every step before doing it for real with production.