While it may sound appealing, offering only free shipping is not the most cost-effective solution available to you. Receiving an order for free is a privilege in the eyes of buyers, so you should take advantage of it. 

Instead, choose to offer free shipping under certain conditions:
One-time free shipping: set up communication based on one-time free shipping (season, sale, anniversary of your store, etc.). This strategy will allow you to boost your sales by inciting your buyers to place an order by benefiting from a temporary advantage.
Free delivery for your loyal clients : free delivery can be a great reward to offer to your clients  who have already ordered from your shop.