As a business owner, you may want to automatically keep track of customers that utilize free shipping options by assigning their orders a shipping SKU. However, this is not an available feature at this time.
Unsupported Shipping Mapping
It is not possible to assign a SKU to a free shipping method, as there is no cost associated. The assignment of a shipping SKU to an order is triggered by a non-zero shipping charge-- since free shipping has no cost, there is nothing to trigger the integration to assign a SKU.
For any other questions, feel free to submit a support request in the Admin Panel.