Replace Paid Shipping Method With Free Shipping Method WooCommerce [duplicate]

You can use the woocommerce_available_shipping_methods filter to do that. NB: if you have Role Based Shipping plugin you might find that it removes the Free Shipping option under some circumstances; this code puts it back if shopper has a coupon for Free Shipping.

/**
* if Free Shipping method is available, reduce to just Free Shipping
* @param array $available_methods
* @return array
*/
add_filter('woocommerce_available_shipping_methods', function($available_methods) {
    global $woocommerce;

    // check for coupon with Free Shipping
    if (wpse_114581_has_coupon_free_shipping()) {
        // ensure that Free Shipping is in the list of available methods (again -- Role Based Methods may strip Free Shipping for Guests)
        if (!isset($available_methods['free_shipping'])) {
            $available_methods['free_shipping'] = new WC_Shipping_Rate('free_shipping', 'Free Shipping', 0, array(), '');
        }
    }

    // if both Free Shipping and another method, reduce to just free shipping
    if (isset($available_methods['free_shipping']) && count($available_methods) > 1) {
        $available_methods = array('free_shipping' => $available_methods['free_shipping']);

        // change title on Free Shipping method
        $available_methods['free_shipping']->method_title="flat rate shipping (free!)";
    }

    return $available_methods;
});

/**
* check for coupon with free shipping
* @return bool
*/
function wpse_114581_has_coupon_free_shipping() {
    global $woocommerce;

    foreach ($woocommerce->cart->applied_coupons as $code) {
        $coupon = new WC_Coupon($code);
        if ($coupon->is_valid() === true) {
            if ($coupon->enable_free_shipping()) {
                return true;
            }
        }
    }

    return false;
}

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