Karlie here from the Shopify Support Team! There is a discount section built into your Shopify Admin and while you do have the option to offer free shipping to your customers, like so, the discount cannot be customized to only apply to specific collections, I'm afraid. What you could do is use an app to customize your Shopify rates per order or collection. I would specifically recommend looking into the apps; Better Shipping, ShipNerd, and Advanced Shipping Manager. If you have any questions about these apps then you can use the 'Support' section on the app page in the Shopify App Store to reach out to the app developer.
I hope this has answered your question. If not, or if you have any further questions please let me know. I will be happy to help!