PDF Invoice attachment for offline payment

I am seeing this bit of code

        //work only when plugin is enabled

		$plgEnable = XiHelperPlugin::getStatus('pdfinvoice','payplans');



		if($plgEnable == true && ($invoice instanceof PayplansInvoice)){

			list($instance, $attachment) = $this->_attachInvoice($invoice);

			if($attachment){

				$mailer->addAttachment($attachment);

			}

		}

in plugins/payplans/offlinepay/app/offlinepay/offlinepay.php

We have the PDF Plugin enabled (it is working on the site, users can download their own PDF invoices) but for some reason, the offline pay email does not attach the invoice

Can you advise how we can attach the invoice to offline payments?

As an aside, the PDF Invoice plugin is using a very old version of dopdf, and has very limited font support, it could probably do with a rewrite.. in fact it would be nice if you had a backend invoice email / invoice template designer as configuring an invoice layout with mutiple plugins is a nightmare that requires changes to several views.