I'm Ann, from the team here at Shopify :)
The best way to do that would be through an app! Apps are like extra features you can add to your store. In this case, here are some apps that will help you add 'Free Shipping' banner or sticker on your products:
I hope this helps! If you have any more questions or concerns feel free to reach out to us here :)
Have a great one,
Thanks Ann, is there any other way than app? we would like to add it by native coding since we are optimising also the site speed and all these apps on the store font will take time to show and will slow down the website.
You can try our app. Our app helps you directly attach "free shipping" label to product images, so it does not affect your website's loading speed.
Find it on Shopify store or here. Besides consulting our sticker library, you can upload your sticker.
Have a nice day and good luck with your business!