POST a Custom Field through the API

Hi all,

We've got a lengthy setup where orders come from Shopify -> Fishbowl -> ShipStation. We need both order numbers on our packing slips. We've mapped the Fishbowl order number to orderNumber in the API. We can't figure out how to include the Shopify order number though. It looks like the templates have 3 custom fields, but they aren't available through the API.

We're thinking about repurposing the userId property with this value, but we'd prefer a more semantic solution.

Any suggestions?