Have you tried creating a Shipping Profile specifically for your dresses collection? This should allow you to create a dedicated shipping rate for that collection only, as well as for the US region.
You can find instructions on how to create a Shipping Profile in this guide, but your shipping configuration should look something like the one in the image below:
Just a reminder that while you'd like to set specific rates for your US buyers, if you're looking to ship the dresses collection to other buyers outside of the US too, you'll need to include shipping rates for those regions, as seen in the second box of the image above.
Seeing as you're currently working on the shipping settings of your store, I'll leave our shipping resources below for you to take a look at in case you'd like to further optimize your shipping strategies:
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You can either use the native Shipping Profiles feature to do this as explained by @Kimi above. But their are some limitations w.r.t. that method.
1. You cannot blend rates when products from multiple profiles are present in cart. 2. You cannot charge per product shipping. 3. Manually assigning products to shipping profiles can be cumbersome if you have a large catalogue. 4. I don't think you can add rates by % in native shipping profiles. You can only add fixed values.
A better solution would be to use an app like ShipMagic (free app). You can easily do all the things you mentioned. We also provide free setup of the app.