First free shipping method has to be enabled with option "A valid free shipping coupon"…
Then you need to set the desired coupon codes with option "Allow free shipping" enabled.
The following code will set all shipping methods costs to zero when a valid coupon code (with option "Allow free shipping" enabled) will be applied.
Update: Hide "Free shipping" method and append shipping label titles with "(free)
"
add_filter( 'woocommerce_package_rates', 'coupon_free_shipping_customization', 20, 2 );
function coupon_free_shipping_customization( $rates, $package ) {
$has_free_shipping = false;
$applied_coupons = WC()->cart->get_applied_coupons();
foreach( $applied_coupons as $coupon_code ){
$coupon = new WC_Coupon($coupon_code);
if($coupon->get_free_shipping()){
$has_free_shipping = true;
break;
}
}
foreach( $rates as $rate_key => $rate ){
if( $has_free_shipping ){
// For "free shipping" method (enabled), remove it
if( $rate->method_id == 'free_shipping'){
unset($rates[$rate_key]);
}
// For other shipping methods
else {
// Append rate label titles (free)
$rates[$rate_key]->label .= ' ' . __('(free)', 'woocommerce');
// Set rate cost
$rates[$rate_key]->cost = 0;
// Set taxes rate cost (if enabled)
$taxes = array();
foreach ($rates[$rate_key]->taxes as $key => $tax){
if( $rates[$rate_key]->taxes[$key] > 0 )
$taxes[$key] = 0;
}
$rates[$rate_key]->taxes = $taxes;
}
}
}
return $rates;
}
Code goes in function.php file of your active child theme (or active theme).
Tested and works. It should works also for you.
Sometimes, you should may be need to refresh shipping methods:
1) Empty cart first.
2) Go to shipping Zones settings, then disable/save and re-enable/save the related shipping methods.