If booking via Expedia, then your payment is just processed via your creditcard (e.g. after the payment you don't have to worry anymore), so no problem if you book via this app.
However, if booking via their own website, then your payment isn't really processed and claimed by the hostel. When you want to check in, you are told to pay for the booking at the reception, and you are told that the original payment for the booking isn't actually a payment but a failsafe for no shows. When you then pay for your booking, then your the original booking payment will be cancelled and the money returns to the creditcard/account.
This is a really weird and inconvenient process, because [1] you can only pay by debit card at the reception (no cash, cc), [2] if someone else paid for the booking/trip, the actual guest might not even have credit to pay for the room, and [3] the cancellation of your booking payment is done manually and therefore prone to human error.
Imagine that someone else pays for your booking, e.g. your boss or parent, and then you arrive at the hostel and suddenly you have to pay there, but you didn't bring any credit because the booking and payment are supposed to be done already. Then, you will not be able to check in because you can't 'pay' for the booking, while the original booking payment is not cancelled because you didn't check in.
Also, they charge you €5 for a small can of supermarket beer (€1 or less around the corner).