You can create several rules. Each rule has own restrictions and priotity. If all rules aren't validate than plugin will display all old default shipping methods and fees.
You did rule for USPS Priority Mail. This rule changes rate for this shipment method. If you have another shipment methods plugin will leave them with old values.
Take one rule and open it. You will see main settings on the first tab.
You can select the name and priority of the rule
-carriers (you can select several ones, leave empty for all carriers)
-shipping methods (you can set several ones, each shipping method is in the new line, leave empty for all shipping methods)
- the minimum and maximum weitgth of the order (example for all order with weigth more 10lb)
- the minimum and maximum sub total for the order (example for all order with sub total more $150.00)
- store mapping
- ACL (customer roles)
- apply this rule if customer has a discount or skip the rule in another way
Open the second tab you can set the start and end date or time of activity of this rule.
You can configure Free Shipping for all orders from weekends.
Open the third tab you can set the list of coupon codes of this rule.
If customer used the coupon code than he will have Free Shipping
Open the fourth tab you can set the method for new rate for this rule.
You can configure new rates for this rule (Replace, Discount, Surcharge).
Open the fiveth tab you can set the list of products (with product attributes) for this rule.
If customer purchase one product from this list he will get a discount.
Open the sixth tab. You can complicated restrictions and add several restrictions in one group.
You can add restriction using billing and shipping address.
You can add restriction using checkout attributes.
You can add restriction using order amounts.
You can add restriction using product specififcations.
You can add several restrictions.