Does the SmartPost API support reference lines?
Yes, although they're not called reference lines in the API.
FedEx SmartPost supports using the Shipment Options
print_custom_3 on a label. The maximum length of each can be 25 characters on the label. There is also room on the label for the Shipment Option invoice_number with a maximum length of 15 characters. You can read more about setting our Shipment Options in our documentation here.