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 send shipment notification when you've only partially shipped it. If you force SKULabs to send shipment notification for a partially shipped order, Amazon will interpret this as a complete shipment. Do not manually submit shipment notification or use shipping settings to force SKULabs to send shipment notification unless you want your Amazon orders to be closed out as shipped before you're finished fulfilling the order.