If your speaking about delivery rates then normally shipping discount rates are discussed with your provider(s) of choice on a yearly basis based upon expected delivery volumes. You may also coordinate with Fulfillman. They can process multiple orders and delivers it fast bulk shipments using their own Private Line.
Once negotiated, you can have your dropshippers use your shipping account numbers to process your shipments so you can obtain the worked out discount rates on a per delivery basis.
In relation to normal decrease ship agreements: the vendor normally works out a margin based upon expected sales. The vendor/dropshipper will certainly nonetheless maintain the right to size business with the seller need to fulfil their expectations.