Hi,
In the settings page of the PDF invoice plugin, you have a setting to select the layout to be used for the invoice.
Each one corresponds to a file in the folder plugins/hikashop/attachinvoice/attachinvoice/
So you can access that folder via FTP, and copy the file corresponding to the invoice you're using to the folder media/com_hikashop/plugins/
Then, you can edit that layout copy which will act as an override of the default layout file.
There, you should find the line:
<div class="invoicetitle"><?php echo $number.$order->order_invoice_number; ?></div>
Which adds the invoice number to the invoice.
So you can add a similar line for the order id. For example:
<div class="invoicetitle">Order id:<?php echo $order->order_id; ?></div>
Note that the Mollie plugin on our website is developed by SkySpider (
skyspider.com.au/#contact
) so you should contact him directly regarding your second question. However, if I'm not mistaken, the plugin should change the status of the order from "created" to "confirmed" after the payment is received (and that's how you know, on HikaShop's side, whether an order is paid or not). And the invoice number is generated when the order is confirmed.
So the workflow should be: checkout -> order created -> plugin sends the information about the order to Mollie -> payment is made -> order is confirmed -> invoice number is generated.
So, it should be impossible for the plugin to provide the invoice number to Mollie as the invoice doesn't yet exist.