When you consider all of the costs involved in maintaining your own warehouse and staff, as well as the new options a order fulfillment center may be able to provide you, you will be able to justify the fact that you will have indirect control over the order fulfillment process.
Options such as fixed costs, and flat-rate shipping, will help you to roll these expenses into your cost model. Especially if you ship sufficient volume to allow your fulfillment company to justify giving you a large discount.
If you weigh the costs of employees, benefits, insurance, taxes, unemployment, facilities, etc, to the fixed cost model provided by an order fulfillment provider, you should quickly see that making the switch is justified. You can often save 20-50% over your existing expenses when you compare the costs of outsourcing, and the costs of processing your own orders in house.
The inverse is true as well. If you are running a ecommerce company all by yourself, or with the help of friends and family, and your costs are down, but you are spending all of your time shipping out orders, purchasing packaging supplies, and reviewing your shipping records. Now, imagine if you took all the people that are working these remedial tasks, and turned them into your marketing / sales team. Send them to trade shows, have them make sales calls, have them negotiate better rates for the products you are selling…