Partial Fulfillments with Shipstation
Shipstation, by default, ignores any pre-existing fulfillments when it pulls in an order from Shopify. This means that if you have orders that have been partially fulfilled in Shopify, the order may appear in ShipStation has not having been fulfilled at all--creating the risk that you will double fulfill an order and send out too much product by accident.
ShipStation recently added a setting that tells it to respect partially and fulfilled line items in an order. This requires that you enable the Use Fulfillable Quantity setting in your ShipStation Shopify settings.
ShipStation's description of the setting is:
Use Fulfillable Quantity
By default, ShipStation will import the line items and quantities from when a Shopify order was originally placed. If this setting is enabled, ShipStation will instead refer to the fulfillable quantity value from Shopify.
The fulfillable quantity value in Shopify is updated whenever orders are fulfilled within Shopify, when orders are partially cancelled, or when the order's line items are edited within Shopify. Enabling this setting allows the line items in ShipStation to updated based on the actions taken in Shopify.
Please see ShipStation's documention: https://help.shipstation.com/hc/en-us/articles/360026141491-Shopify
Please note that ShipStation still may not sync order updates if the order is older than 14 days, including fulfillments.