Example of adding fixed g:shipping tag to Google Product Feed

functions.php

<?php

add_filter( 'woocommerce_gpf_feed_item_google', function ( $feed_item, $wc_product ) {
    $feed_item->additional_elements['shipping'] = [
        [
            'country' => 'GB',
            'region'  => 'Cumbria',
            'service' => 'Royal Mail',
            'price'   => '1.99 GBP',
        ]
    ];

    return $feed_item;
}, 10, 2 );