With drop shippers you just need to set your origin city, and origin zip/postal code to the one where your product is actually shipping from. This does not use the drop shippers rates - but does the lookup based on the information within the shipper extension. You would need to offer only the same level of service that the drop shipper uses eg. if drop shipper does not allow for express shipping, then you should not enable that service within the shipping extensions on your cart.