WooCommerce Checkout: Add multiple custom fields to billing area

functions.php

// https://businessbloomer.com/woocommerce-add-shipping-phone-checkout/
// Year
add_filter( 'woocommerce_checkout_fields', 'bbloomer_billing_year_checkout' );
 
function bbloomer_billing_year_checkout( $fields ) {
   $fields['billing']['billing_year'] = array(
      'label' => 'I am over 18',
      'type'  => 'checkbox',
      'required' => true,
      'class' => array( 'form-row-wide' ),
      'priority' => 26,
   );
   return $fields;
}
  
add_action( 'woocommerce_admin_order_data_after_shipping_address', 'bbloomer_billing_year_checkout_display' );
 
function bbloomer_billing_year_checkout_display( $order ){
    echo '<p><b>Billing Year:</b> ' . get_post_meta( $order->get_id(), '_billing_year', true ) . '</p>';
}

// Organization
add_filter( 'woocommerce_checkout_fields', 'bbloomer_billing_org_checkout' );
 
function bbloomer_billing_org_checkout( $fields ) {
   $fields['billing']['billing_org'] = array(
      'label' => 'Organization',
      'type'  => 'text',
      'required' => true,
      'class' => array( 'form-row-wide' ),
      'priority' => 27,
   );
   return $fields;
}
  
add_action( 'woocommerce_admin_order_data_after_shipping_address', 'bbloomer_billing_org_checkout_display' );

function bbloomer_billing_org_checkout_display( $order ){
    echo '<p><b>Billing Year:</b> ' . get_post_meta( $order->get_id(), '_billing_org', true ) . '</p>';
}

// Sales
add_filter( 'woocommerce_checkout_fields', 'bbloomer_billing_sales_checkout' );
 
function bbloomer_billing_sales_checkout( $fields ) {
   $fields['billing']['billing_sales'] = array(
      'label' => 'How many will sell these products?',
      'type'  => 'text',
      'required' => true,
      'class' => array( 'form-row-wide' ),
      'priority' => 28,
   );
   return $fields;
}
  
add_action( 'woocommerce_admin_order_data_after_billing_address', 'bbloomer_billing_sales_checkout_display' );

function bbloomer_billing_sales_checkout_display( $order ){
    echo '<p><b>How many will sell these products?</b> ' . get_post_meta( $order->get_id(), '_billing_sales', true ) . '</p>';
}


// Contact person
add_filter( 'woocommerce_checkout_fields', 'bbloomer_billing_contact_checkout' );
 
function bbloomer_billing_contact_checkout( $fields ) {
   $fields['billing']['billing_contact'] = array(
      'label' => 'Contact person from our company?',
      'required' => true,
      'class' => array( 'form-row-wide' ),
      'priority' => 28,
      'type'  => 'select',
      'options'  	=> array( // options for <select> or <input type="radio" />
	      ''    		=> 'Please select', // empty values means that field is not selected
		            	'ingen_kontakt_person'	=> 'No contact person', // 'value'=>'Name'
			                          'Regine'	=> 'Employee 1',
			                         'mei_mei'	=> 'Employee 2'
		)
      
   );
   return $fields;
}
  
add_action( 'woocommerce_admin_order_data_after_billing_address', 'bbloomer_billing_contact_checkout_display' );

function bbloomer_billing_contact_checkout_display( $order ){
    echo '<p><b>Contact person from our company?</b> ' . get_post_meta( $order->get_id(), '_billing_contact', true ) . '</p>';
}