How to make the Shipping Rules extension consider the cart subtotal including tax?
If a subtotal condition is used in one of your shipping rules, it's possible to configure the extension to provide the calculations based on subtotal with tax. Navigate to Stores → Configuration → Amasty Extensions → Shipping Rules and enable the Include Tax in Subtotal setting:
Having set this setting to Yes, you'll see that rules will operate subtotal + tax after that.
Moreover, starting from version 2.9.1, the extension introduces a new Subtotal (Incl. Tax) condition. Now you can specify the necessary subtotal value, inclusive of tax, for each individual rule. To configure it, navigate to Sales → Shipping Rules → edit or create a rule → Conditions tab.