Automatically Send Invoice After Set to Pending

functions.php

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

function maybe_send_invoice_notification( SI_Invoice $invoice, $status = '' ) {
	if ( SI_Invoice::STATUS_PENDING === $status ) {
		$client = $invoice->get_client();
		if ( ! is_a( $client, 'SI_Client' ) ) {
			return;
		}
		$recipients = $client->get_associated_users();
		if ( empty( $recipients ) ) {
			return;
		}
		do_action( 'send_invoice', $invoice, $recipients );
	}
}
add_action( 'si_invoice_status_updated', 'maybe_send_invoice_notification', 10, 2 );