Compatibility with WooCommerce free shipping

I have a store set up that has free shipping applied for orders over $150. I also offer a 20% discount for cash and bank transfer payments.
If the subtotal order amount is $170, the free shipping becomes available. The discount is applied after shipping options and the final order amount is $140 and the free shipping remain available, this causes that the customer gets the discount and the free shipping in an order under $150.
To avoid this issue I need to apply the discount before shipping options.