Is there a long term commitment?
Some companies try to lock you in to long term contracts. Long term can be suitable, but on occasion you will get locked in only to find the service is terrible. If a fulfillment company requires a long term commitment, be sure that there is a way out of the contract.
Better yet, find fulfillment companies that are committed to service and have short term renewable agreements or no commitment at all. If they are good at what they do and are committed to customer service, then they won't require a long term commitment.