Thank you both! I was able to return the mistaken purchase and get a credit back. I had to log into Audible's website and go to Account->Purchase History (which presented its own temporary issues) but I finally got there.
EDIT: and having done all that, I managed to find my way to my Purchase History withIn the Audible app.
I've seen no return buttons anywhere in my library. Only links to return this item from the purchase history in the account submenu.