If this ever happens to you, immediately call bank, say “Electronic transaction posted in error.”, specifically identify the transaction, and ask what address the bank takes Regulation E written complaints at. If the CSR doesn’t know that answer, their supervisor does, or in the alternative FedEx HQ addressed to chief counsel or head of compliance. The letter just needs to state transaction details, date you first called them, and your desired action (“Credit me back $X.”), but it’s marginally more effective to say Regulation E in it since that will put the fear of God into whomever opens it.
You’ll get the money back.