Adds VAT and SSN fields in WooCommerce workflow

functions.php

// Add VAT and SSN fields in billing address display
add_filter( 'woocommerce_order_formatted_billing_address', 'custom_add_vat_ssn_formatted_billing_address', 10, 2 );
function custom_add_vat_ssn_formatted_billing_address( $fields, $order ) {
	$fields['vat'] = $order->billing_vat;
	$fields['ssn'] = $order->billing_ssn;

	return $fields;
}

add_filter( 'woocommerce_my_account_my_address_formatted_address', 'custom_my_account_my_address_formatted_address', 10, 3 );
function custom_my_account_my_address_formatted_address( $fields, $customer_id, $type ) {
	if ( $type == 'billing' ) {
		$fields['vat'] = get_user_meta( $customer_id, 'billing_vat', true );
		$fields['ssn'] = get_user_meta( $customer_id, 'billing_ssn', true );
	}

	return $fields;
}

add_filter( 'woocommerce_address_to_edit', 'custom_address_to_edit' );
function custom_address_to_edit( $address ) {
	global $wp_query;

	if ( isset( $wp_query->query_vars['edit-address'] ) && $wp_query->query_vars['edit-address'] != 'billing' ) {
		return $address;
	}

	if ( ! isset( $address['billing_vat'] ) ) {
    	$address['billing_vat'] = array(
        	'label'       => __( 'VAT', 'your-domain' ),
            'placeholder' => _x( 'VAT', 'placeholder', 'your-domain' ),
            'required'    => true, //change to false if you do not need this field to be required
            'class'       => array( 'form-row-first' ),
            'value'       => get_user_meta( get_current_user_id(), 'billing_vat', true )
        );
    }

    if ( ! isset( $address['billing_ssn'] ) ) {
    	$address['billing_ssn'] = array(
        	'label'       => __( 'SSN', 'your-domain' ),
            'placeholder' => _x( 'SSN', 'placeholder', 'your-domain' ),
            'required'    => true, //change to false if you do not need this field to be required
            'class'       => array( 'form-row-first' ),
            'value'       => get_user_meta( get_current_user_id(), 'billing_ssn', true )
        );
    }

    return $address;
}

add_filter( 'woocommerce_formatted_address_replacements', 'custom_formatted_address_replacements', 10, 2 );
function custom_formatted_address_replacements( $address, $args ) {
	$address['{vat}'] = '';
	$address['{ssn}'] = '';

	if ( ! empty( $args['vat'] ) ) {
		$address['{vat}'] = __( 'VAT', 'your-domain' ) . ' ' . $args['vat'];
	}

	if ( ! empty( $args['ssn'] ) ) {
		$address['{ssn}'] = __( 'SSN', 'your-domain' ) . ' ' . strtoupper( $args['ssn'] );
	}

	return $address;
}

add_filter( 'woocommerce_localisation_address_formats', 'custom_localisation_address_format' );
function custom_localisation_address_format( $formats ) {
	$formats['IT'] .= "\n\n{vat}\n{ssn}";

	return $formats;
}

add_filter( 'woocommerce_admin_billing_fields', 'custom_admin_billing_fields' );
function custom_admin_billing_fields( $fields ) {
	$fields['vat'] = array(
		'label' => __( 'VAT', 'your-domain' ),
		'show'  => true
	);

	$fields['ssn'] = array(
		'label' => __( 'SSN', 'your-domain' ),
		'show'  => true
	);

	return $fields;
}

add_filter( 'woocommerce_found_customer_details', 'custom_found_customer_details' );
function custom_found_customer_details( $customer_data ) {
	$customer_data['billing_vat'] = get_user_meta( $_POST['user_id'], 'billing_vat', true );
	$customer_data['billing_ssn'] = get_user_meta( $_POST['user_id'], 'billing_ssn', true );

	return $customer_data;
}

add_filter( 'woocommerce_customer_meta_fields', 'custom_customer_meta_fields' );
function custom_customer_meta_fields( $fields ) {
	$fields['billing']['fields']['billing_vat'] = array(
		'label'       => __( 'VAT', 'your-domain' )
	);

	$fields['billing']['fields']['billing_ssn'] = array(
		'label'       => __( 'SSN', 'your-domain' )
	);

	return $fields;
}