This extension enables you to sell voucher as product. This brilliant extension is made by shambhal who has maximum opencart reviews on freelancer.com
This extension is ocmod based so no core changes. It works well with opencart default structure and theme.
Custom theme may need to be tweaked and developer is not responsible to fix for their theme.
If other extensions invalidate this , developer cannot be held responsible. Developer may charge extra to make it compatible with other extensions , though chances are meager as this is a unique extension.