Out-of-stock refund hassle

I payed for a phone on the websit (1 in stock) and then received an e-mail stating that they are actually out of stock. They gave me the option to pay extra and get a more expensive phone or to get a refund.

Turns out that they now "require a stamped Bank letter to confirm the account number as well".

They are wasting a lot of my time because of a mistake on their side.