You can code this directly into your theme, without using a store facing app that could potentially slow your load speed.
Here’s how:
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Set a banner to show either in the header or on the product page, saying something like “Free shipping at $50” or “Add $24.65 to your cart to qualify for FREE shipping”.
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For the logic, you would have some JavaScript on all pages of your site that banner appears to check the cart contents using the ajax API (https://shopify.dev/docs/api/ajax).
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You would retrieve the cart contents after each action on the cart (adding / subtracting product).
The implementation would depend how your theme is set up, but a Shopify developer will be able to investigate and provide a solution.
If you can find a low priced app to do it, that might be worth it while you’re testing the feature. Something like this though I would recommend coding directly into the theme eventually.
If you don’t have a go-to developer and need help finding one feel free to contact my team at orderautomator.com/support. We make Shopify apps, but have a network of Shopify experts we work with, and can recommend you a developer if you need one.