Mass Invoices Updater, Example updates GST and Tax

functions.php

<?php // don't include this in your functions.php, since the start of the file should already have it

function _maybe_update_invoice_line_items_with_gst( $invoice_id ) {
	$invoice = SI_Invoice::get_instance( $invoice_id );
	if ( ! is_a( $invoice, 'SI_Invoice' ) ) {
		return false;
	}
	$line_items = $invoice->get_line_items();
	foreach ( $line_items as $position => $data ) {
		$line_items[ $position ]['tax_gst'] = 5;
		$line_items[ $position ]['tax'] = 0;
		if ( 0 === (int) $data['rate'] && 0 === (int) $data['qty'] ) {
			unset( $line_items[ $position ] );
		}
	}
	$invoice->set_line_items( $line_items );
	return true;
}

function run_si_gst_updater( $offset = 0 ) {
	$args = array(
		'post_type' => 'sa_invoice',
		'post_status' => 'any',
		'posts_per_page' => 20,
		'fields' => 'ids',
		'offset' => $offset,
	);
	$post_ids = get_posts( $args );
	if ( empty( $post_ids ) ) {
		wp_die( 'importer possibly done.' );
		exit();
	}
	foreach ( $post_ids as $post_id ) {
		error_log( 'post_id: ' . print_r( $post_id, true ) );
		_maybe_update_invoice_line_items_with_gst( $post_id );
	}

	$new_offset = $offset + 20;
	wp_redirect( add_query_arg( array( 'gst_updater' => true, 'offset' => $new_offset ) ) );
	exit();
}

// go to url http://yoursite.com/wp-admin/admin.php?gst_updater=1
if ( is_admin() && current_user_can( 'edit_posts' ) ) {
	if ( isset( $_GET['gst_updater'] ) && $_GET['gst_updater'] ) {
		$page = ( isset( $_GET['offset'] ) ) ? $_GET['offset'] : 0 ;
		run_si_gst_updater( $page );
	}
}