Computicket- unwilling to refund without cancelation fee

Purchased tickets this morning at 10am from Computicket for the Matilda show. Unfortunately tickets were added to my basket twice and I paid without realising this. Phoned computicket at 12 and the consultant informed me that I have 1 hour to cancel an order.

Requested a cancelation from the event organisers and they indicated that I have to pay a 15% cancellation fee.

I believe this is extremely unreasonable as I cancelled this more than a month before the show and this show will probably be sold out in anyway.

I support Computicket often however I will think twice in future.