If Checked then value else return 0

image

function calculate_custom_text_fee( $cart_object ) {
    foreach ( $cart_object->get_cart() as $cart_item ) {

            // get the custom pricing for this product
            $pricing_custom = get_post_meta( $cart_item['product_id'], '_number_field_1', true );


            // get product price
            $price = floatval( $cart_item['data']->get_price() );

            // set new price
            $cart_item['data']->set_price( $price + $pricing_custom );
    }
}
add_action( 'woocommerce_before_calculate_totals', 'calculate_custom_text_fee', 99, 1 );

But How can we ensure through the coding that only when the checkbox is ticked should this give a +ve quantity else 0→

$pricing_custom = get_post_meta( $cart_item['product_id'], '_number_field_1', true );

What does get_post_meta return if the thing isnt checked?