Premium store is a *******!

Shocking! Asked to check a powerbank on my iPhone to see if it was working appropriately, and was told that I have to pay first before opening it. The assistant said if a problem then they will just refund me. (Noting there was a previously opened box in the display). But so I paid, and wasn’t happy with the item, so asked for a refund. Charged 20% “handling fee” to get my money back. No help from manager, customer care seemingly sympathetic but unable to help, and no response from the email they gave me.