So far I've found Shipwire, but they seem pricey so I'm looking for others to compare prices.
It would depend on a number of variables, what your shipping, to and where from, do you want it to integrate into an existing store, or are you building a store that you want to integrate this with, which platform are you planning to use or using?
That all said, here are a couple that may be of interest to you, to get you started:
http://fulfillrite.com/
http://alivefulfillment.com/
Also, whether you use shopify or not, there app directory is also a good place to look to start finding solutions and service providers:
https://apps.shopify.com/search/query?utf8=%E2%9C%93&q=order+fulfillment
Hope this helps.
-Branden
