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<?php 

// the code below goes in your functions.php file

function my_custom_prepare_shop_order_object($response, $order, $request) {
  if( is_pos() ) {
    $data = $response->get_data();
    $data['regular_price'] = get_post_meta( $data['id'], 'wholesale_customer_wholesale_price', true );
    $response->set_data($data);
  }
  return $response;
}

add_filter( 'woocommerce_rest_prepare_product_object', 'my_custom_prepare_product_object', 20, 3 );
add_filter( 'woocommerce_rest_prepare_product_variation_object', 'my_custom_prepare_product_object', 20, 3 );