Has it been more than 24 hours since you ordered? If so, contact the merchant, explain what happened, and politely request redelivery of the item. If it has been less than 24 hours, please be patient and check again later. If the merchant doesn't respond, you can submit a support case requesting redelivery. Make sure to include the order number and the exact name of the undelivered item(s).
Slow deliveries may currently be more noticeable because you now have a cart to which you can add multiple items. (This wasn't the case with the old Xstreet, which closed for purchases in October 2010.)