Click "ADVANCED OPTIONS" on the left product sidebar
In the Channel Overrides slider that appears => Check the "Free Shipping" box => Click "UPDATE & PUSH"
**Make sure the selected Carrier / Service Levels qualify given the shipping dimensions of the SKU. For example: if you have USPS First Class selected but the SKU is over 1lb, then eBay will consider USPS First Class the only service level that qualifies for free shipping and since First Class shipments must be under 1lb, Free Shipping will be disregarded and the listing will not be Free Shipping.