If Unleashed is sending down a zero value for the shipment weight, you can choose to have Shiptheory to calculate the total shipment weight from the shipment product weights stored in Shiptheory.
You can enable this setting in the advanced settings part of your unleashed Integration.
Note: If you are downloading shipment product weights, and do not want to use the values being downloaded with the shipment, you need to select the option to disable the product weight override. See this article for more information.