You may veiw your deliverys by opening the 'stockroom' tab, and then the 'deliverys' one. Here you will find a list of which items you are currently ordering, and how lon it will be til they arrive. In order to unlock more delivery slots, you may need to add more Neighbours, and then purchas it with MallDollars. If you don't want to add more neghbours, or don't have enough MallDollars, you may purchas the new delivery slot for an amouint of MallCredits.
When you order an item from a collection, it will take a ceratian amount of time for it to be delivered to your Inventory. If you do not have any spare delivery slots, you won't be able to order any more new items from the Catalogue untill your current pending orders have been compleated.
In order to maximise your profit margins, try to choose large and bulk-buy deliveries, even though they may take longer to arrive. When chooseing to purchas 10 or more items in a single delivery, you will have to pay less Mall Dollars per item to stock your stores.
This not works on the recent update of Mall World.