I'm looking for a plugin where the buyer can add all of the items into their cart and request a manual combined shipping rate before processing their payment. I need to be able to manually adjust the shipping rate, yet have the order marked as "pending" so that way the buyer's items aren't sold to someone else in the process.
If you have a FedEx, USPS, DHL or UPS account enter your credentials in
System->Configuration->Shipping Method and you will get a real quote based on weight. That's the normal behavior, you don't need a plugin.
You can see the configuration instructions here : http://merch.docs.magento.com/ce/user_guide/Magento_Community_Edition_User_Guide.html#shipping/online-rates.html%3FTocPath%3DShipping%7CBasic%2520Shipping%2520Methods%7C_____5