How can I tell what order data is available to map?

You can see what order data fields are available to map from within the Order Mappings tool in your FarApp account. 
https://app.farapp.com/settings#/marketplace_cart_settings/order_sync/order_mapping

1. Click the green Add Mapping button.
   
   
   
2. Select the NetSuite field you want to map to and click the Add Mapping button.
3. The mapping will be added to the end of your list of mappings.
4. Click Close to close the window.
5. Locate your new mapping in the list of mappings, change the Mapping Type to Order Header with Translation.
6. For the Storefront/Fixed Value field, select the Custom Field option from the dropdown list.
7. Enter an account and Order ID (with the applicable data) in the window that appears. Then click the View Field Data button.
   
   
   
8. Click the + button next to the order data field you want to map.
9. If you want to set a default value, enter it in the Default Value field. If you want to add logic to the mapping, select Add Row.
   
   
10. Enter the marketplace/cart's value to look for in the left column. Enter the NetSuite value in the right column.
11. If there are additional rules you want to add, select Add Row again. When you are done, click Save. Then Close.
12. Click Save on the Order Mappings page to save all your mapping changes.