Free shipping above $65 is calculated with your subtotal AFTER any discount is applied and BEFORE taxes.
If your total before adding a discount is just above $65, then adding a discount will drop your subtotal to below the free shipping threshold. Adding a few extra items usually remedies this.
Also note there are certain products that require a shipping fee: the Monster Femur Bone and the 32-36" Bully Stick Cane. This information can be found on the products primary image and written in the description.