In order to assign a specific shipping price to a single item you will need to create a two overrides. One for the price you want to charge for shipping and one to remove the value shipping.
1. Create an shipping override for the price you want to charge (This is a flat fee)
2. Create a shipping override to remove Free Value Shipping for this one item
3. After you have created both you will need to export this product to Walmart again for the changes to take place.