Unfortunately, Amazon does not offer partial shipment support through the Amazon MWS (Merchant) APIs.

This restricts our ability to tell your Amazon customers which items were shipped in which package like you see within SKULabs.
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When you complete shipment of an Amazon order we'll send the tracking for the most recent package to Amazon.

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Note: By default SKULabs will not mark an amazon order as shipped when you've only partially shipped it. If you force SKULabs to mark a partial order as shipped, Amazon will interpret this as a complete shipment. Do not manually mark as shipped or use shipping settings to force SKULabs to mark as shipped unless you want your Amazon orders to be closed out as shipped before you're finished fulfilling the order.