Add a checkbox below a specific shipping method in WooCommerce 3
Using the $method argument that is an instance of the WC_Shipping_Rate Object you can target any shipping method Id using get_id() method, like in this example:
add_action( 'woocommerce_after_shipping_rate', 'checkout_shipping_additional_field', 20, 2 );function checkout_shipping_additional_field( $method, $index ){ if( $method->get_id() == 'flat_rate:12' ){ echo '<br> <input type="checkbox" name="shipping_custom_1" id="shipping_custom_1" value="1" class="shipping_method shipping_custom_1"> <label for="shipping_custom_1">Custom label</label>'; }}Code goes in function.php file of your active child theme (or active theme). Tested and works.
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