Sliced Invoices: example of how to add "non-cloneable post metas"

functions.php

add_filter( 'sliced_invoices_non_cloneable_post_metas', 'si20220524_add_non_cloneable_post_metas' );
function si20220524_add_non_cloneable_post_metas( $non_cloneable_post_metas ) {
	$non_cloneable_post_metas[] = '_sliced_invoice_created';
	$non_cloneable_post_metas[] = '_sliced_invoice_due';
	return $non_cloneable_post_metas;
}