Well, if it were up to me, my ending would be that when you purchase the final antivirus upgrade, ads fill the entire screen so that when they're cleared, the entire progress of the upgrades would be lost. Sounds crazy, but after a player downloads all files, is there even a reason to continue upgrading? (besides, of course, just for the sake of completion). Once a player downloads every document, each successive upgrade lessens the difficulty of the levels, making them easier and easier, until the fun of racing the timer is lost. So clearing the upgrades would reset the difficulty, and allow replay-ability to your levels. Also, it would be very funny. :p
Some context: I played until I unlocked all the upgrades (to the $50 one); and I also think the mechanic of clicking ads with a timer is already very fun already. These notes aren't 'criticisms; I just had some ideas... ;)
Or there's always the simple 'end credits' method, where you thank the player for playing and return them to the main menu; you could insert this after a player completes all downloads.
Thanks for your time!
PS- a bit of a random thought, but for the downloads, would you consider adding a coarse, but funny, drawing of the file we just downloaded; as a joke, of course. This could add a layer of comedy and distinction to each stage. Just some thoughts.