Amazon PO number requirement

Hi,

I need to have the customer's PO number referenced on the label and packing list for Amazon orders where the customer used a PO#. This is a requirement for Pro sellers and we just want to make sure we're following the rules. We would still need the Amazon order ID# referenced, as it currently is.

I see where to set up the shipment defaults for our Amazon store, but not sure what selection I should choose for what would be a PO field.

Thanks!