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