global $post; in WooCommerce

by

You should avoid using global variables where possible. For metaboxes you should use the $post variable that is passed to the callback function instead:

add_meta_box( 'supplier_package_box', __( 'Supplier', 'supplier' ), 'populate_meta_box' );

function populate_meta_box( $post ){
    print_r($post);
}