For more details see the Shipping Rules extension page.
Manage live shipping rates according to your needs: surcharge, deduct or override rates, provided by different carriers.
Oftentimes it happens that shipping rules don't work because of incorrectly specified conditions. See the example below The problem with these conditions is that Magento tries to find a product with SKU “sku1,sku2,sku3” in the cart, which is wrong. Of course, we wanted it to look for one of the products from the list: sku1, sku2 or sku3. This is how rule should've been configured for this requirement
Please see the example rule setup below This rule will work like this: when there are 5 or more sku1,sku2 or sku3, replace shipping rate only for sku1,sku2 and sku3 with $0. If there are any other products in the cart when the rule activates, their shipping rates won't be altered.
When you have multiple active shipping rules at the same time and they are set to Replace original shipping rates then you may end up in the situation like below:
Confusing, isn't it? Certainly, you don't want something like this to happen so you should make sure all your rules will be applied only when necessary. You can use Address Conditions to limit the cases when the rule can be applied.
Shipping Rules are set up in Sales > Shipping Rules