Once you place your order there is a 15 minute window where you can cancel at the push of a button on your order summary page. We only take payment once this cancel button disappears from this page. Once this point has passed if you wish to cancel your order, you will need to contact the retailer as soon as possible. You can do this from your order page, as below;
If a retailer has an available phone number this might be the fastest way to reach them, otherwise just use the ticket submission facility. So long as the order has not been dispatched the retailer will be able to confirm cancellation with you.