Did you do a preorder for anything? I often pre order books and films when they are first announced, they are listed in your amazon account by the date ordered which might have been a year ago.
I know someone who was charged for Amazon Prime with no knowledge of ordering it but I think it was £97 they took from them, it was refunded after she contacted them.
Glad it's sorted.
It's a shame your wife doesn't also have an AB account- you might have got your answer earlier.
Regarding emails from paypal (Eve's post) , I sometimes get ones saying I've been invoiced for something (last week was a subscription to an X box service) but these tend to be spam emails which don't mention my name and when i hover over the paypal address it's often somewhere in russia or rumania