If its for sales through Amazon (or even elsewhere) FBAPrep.com is one of the best out there. They can handle importing, preparation (repackaging, barcoding, etc.) and then final shipment. Plus, they know the special requirements of Amazon and other third party marketplaces.
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?
Also, whether you use shopify or not, there app directory is also a good place to look to start finding solutions and service providers:
Hope this helps.