woo-reminders.php

<?php
function email_past_due() {

	global $wpdb;

	$mailer     = WC_Emails::instance();
	$date       = date( "Y-m-d H:i:s", current_time( 'timestamp' ) + 86400 * 7 );
	$due_orders = $wpdb->get_col( $wpdb->prepare( "
		SELECT 	posts.ID
		FROM 	{$wpdb->posts} AS posts
		WHERE 	posts.post_type = 'shop_order'
		AND 	posts.post_status = 'wc-pending'
		AND 	posts.post_date < %s
	", $date ) );

	if ( $due_orders ) {
		foreach ( $due_orders as $due_order ) {
			$order = wc_get_order( $due_order );
			$mailer->customer_invoice( $order );
		}
	}

}
?>