Hostelworld and Santa Monaca Charged Incorrectly and Didn’t Issue Refund

I made a reservation through the Hostelworld app for Hostel Santa Monaca in Florence for August 14-15. I was charged €9.50 as a reservation fee, with the remaining €61.86 to be paid during my stay. However, I realized I had booked the wrong dates and emailed the hostel to request a change. They informed me that they had mistakenly charged my account €107.72 and assured me they would issue a refund. Despite their promise, the refund has not been processed, and they have stopped responding to my emails. This situation is unacceptable. I demand an immediate refund of my money and the cancellation of my incorrect reservation. Please resolve this issue promptly!