That script is really old, almost 3 years by the looks of it. System requirements are for PHP 5.2 and MySQL 4.1 which are no longer supported versions. The demo site's last post was from 2012. The forum link is dead. I can't find a single review for the product. For something that old, it's a HUGE red flag!! You'd be insane to drop $395 something like that. Plenty of others to choose from.
I will suggest to use some well known eCommerce solution like OpenCart/Magento - they are comprehensive slutions with plenty of themes as well as extensions. So instead of using an outdated script use some something FREE but efficient solution.
Other alternative would be the good old OSCommecer which has hundred if not thousands of plugins too.