Unfortunately, at the moment there is a glitch in Seller Admin > Shipping Rules, where a UPS rule can apparently only be set for United States but winds up getting created for All Countries, even though it's really only applicable in the cart for destinations in the USA anyway. This means there's no way to create a UPS rule specific to United States that would override any other rule(s) defined for All Countries.
Development has been working on a new version of Seller Admin that should be ready to roll out soon, and this should include a fix for this issue, so you could then make your UPS rule for United States that would override your USPS rule for All Countries.
Meanwhile, the only way to accomplish this would be to set up a UPS rule specific for each US State, since rules for more specific destinations override rules for more general destinations. Fortunately, you can set a rule for one State, hit Submit to save the rule, then just hit your down-arrow key to pick the next State in the menu and click Submit to save the next rule, etc. until you've done all 50 States. A bit tedious, yes, and we do apologize for the inconvenience, but that's what we can suggest until the new Seller Admin goes live with that glitch fixed.
