To create links on your website so that customers can buy products directly, the links to create are:

route to INVOPANEL/order.php?packagename=package name

or

route to INVOPANEL/order.php?packagealias=alias package

In the first option package name should be replaced by the actual name of the package created in the packages.
In the second option should be replaced by the package alias alias you specified in the package (package sections)