I need to write easy php snippet to change shipping cost above cart total

I need to write easy php snippet to change shipping cost above cart total. I need to get shipping costs to 0CZK if cart total is more or equal to 1200CZK (price after using discount coupons). I tried several plugins, but no one fit for my purpose. Best solution for me is if code define shipping cost discount 100% if cart total after using coupon is equal or more than 1200CZK. Please do nto change shipping options etc.. there is one shopping option called Zásilkovna where is logo for updating local pickup centers. We need just to change prices for caert total above 1200CZK.
This is ecomm store: www.femmi.cz
It run on WP - woocommerce

Thanks