Where are you located? (In proximity to my customers)
Due to today's Internet technology and real-time reporting, proximity of your fulfillment house to your customers is more important than the proximity of your fulfillment house to your location. For example, if all of your customers are in the Northeastern United States, then hiring a fulfillment company in California is not a good idea.
As a point of reference, 70% of the US population is located in the Southeast, Northeast and Midwest with California housing approximately another 16%. If your order distribution is spread evenly throughout the country then New York, Pennsylvania or Ohio are good choices to minimize shipping costs.
If most of your customers are in the East then New York, New Jersey, Pennsylvania or Maryland are good choices.
Finally, if most of your customers are in the West then Nevada, California, or Utah are good choices.