You can do this using Shopify's Shipping Profiles feature. In your Shipping & delivery settings, you can set up a custom Shipping Profile for your Collection. You can then add two rates with Price conditions - $5.00 up to a $50.00 price total, and $0.00 for a price total of $51.00 and up.
The way it will work is that if the customer orders $50 or less of the special Collection along with other products, Shopify will blend the $5 rate with the rate for the other products (from your General Profile). Similarly, if the customer orders more than $50 of the special Collection along with other products, they'll only pay shipping costs for the other products. If those are the outcomes you're expecting, then a custom Shipping Profile should be the ideal solution for you.
If your requirements are more complex than that - for example if you want the customer to get free shipping on the entire cart if they order at least $51 of the special Collection, or if you only want to offer free shipping on the Collection when the cart contains only products from that Collection, Shopify Shipping doesn't provide that kind of granular control.
With Intuitive Shipping, you set up a conditional free shipping method to cover these more complex and specific requirements using our Condition system. Our Shipping Scenarios work just like Shopify Profiles, but with far greater control and flexibility. If you find that Profiles don't provide you with enough control, and you'd like to learn more about Intuitive Shipping, please feel free to book a demo with one of our team members.