When viewing an Order it is helpful to be able to know if you have enough inventory to fulfill the Order. You can view the Fulfillability tab, the Fulfillabililty Details will explain which SKU(s) that do not have enough stock to fulfill against.
How is this property accessed via the API?
I'd like a script to take action on orders based on the fulfillability details, specifically "Not Fulfillable Only USPS can ship to a PO Box."
Are you trying to change the Carrier based on that message? If so, you can utilize the Triggers feature. This will allow you to set a Smart Filter on the Fulfillablity Details field to include that message and then the trigger can change the Carrier to USPS. Let me know if that is what you are thinking.
I was thinking a script would be the way to go… Fulfillability Details doesn’t appear to be available as Smart Filterable: