I'm trying to make this Joomla Update system work for paid extensions.
How I'm going about it is as follows:
I'm using Mohameds downloadid.php to populate "extra_query" field in the "_update_sites" table.
after you check for updates joomla looks into your xml file and checks for new versions
then after it sees a new update is available and you click on update
it follows the link in the xml updateserver file adding "?dlid=35423542354" at the end of it
I have download.php file at my end that does the following: GET the variable "dlid" from the url, check it against my database/subscription system and return the file or not