gistfile1.txt

add_filter(
    'woocommerce_get_cart_item_from_session',
    function ( $cart_item, $values ) {

        if ( isset( $values['donation_amount'] ) ) {
            $cart_item['donation_amount'] = $values['donation_amount'];
            $cart_item['is_donation']     = true;

            $cart_item['data']->set_price(
                (float) $values['donation_amount']
            );

            $cart_item['data']->set_virtual( true );
        }

        return $cart_item;
    },
    20,
    2
);


add_action('woocommerce_before_calculate_totals', function ( $cart ) {

    if ( is_admin() && ! wp_doing_ajax() ) {
        return;
    }

    if ( ! $cart || $cart->is_empty() ) {
        return;
    }

    foreach ( $cart->get_cart() as $cart_item ) {

        if ( empty( $cart_item['donation_amount'] ) ) {
            continue;
        }

        $cart_item['data']->set_price( (float) $cart_item['donation_amount'] );
        $cart_item['data']->set_virtual( true );
    }

}, 10 );