Thank you for sharing the screenshots of your shipping rates and the issue you are experiencing. You are correct, as long as the order total is over 50$, based on the shipping rates you have setup, the customer should be seeing the option for free shipping as the default shipping rate at the checkout.
If that is not the case, or if customers are choosing other shipping rates instead, I would recommend going through the checkout (don't complete the sale) and recreate these orders yourself to see what options are coming up. It is possible that your customers are changing the shipping rate as the free rate and the other paid rates will still be showing. If you feel that the shipping experience is not correct, I would recommend contacting our live support for a more in-depth look into what is causing this issue.
To contact our live support please follow this link: Shopify Help Center - Contact Support, sign into your store account, search for your issue and use the contact support button at the bottom of the search results to see all our live support options.
If you want to create some test checkouts using a fake address similar to your customers to simulate the shipping rates you can also share the abandoned checkout URL for us to look at through the forums.
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