Hi,
The above will not add 20 days to the invoice date, it will just display a message.
To add automatically 20 days, you have to edit the view "Order / invoice" via the menu Display > Views and add a line like:
<?php
$dueDate = (int)$this->element->order_created + 1728000;
echo JText::sprintf('MY_PAYMENT_DUE_DATE').': '.hikashop_getDate($dueDate ,'%d %B %Y ');?><br/>
1728000 is equivalent to 20 days in timestamp.
PS: this is just display, there is no automatic management of the due date.
Regarding the Pro Forma Invoice, I have not knowledge about a link to solve this matter.