You installed 0.0.5? If you have, and have also deleted those two mentioned files in /admin, the only thing I could suggest is that you try uninstalling it except for the database. I dont know otherwise, since I'm pretty confident that file_exists works properly. You could, however, try copying and pasting the file_exists check from admin_store to your own test page and try it. If it still gives the error and you still dont believe the files exist, you could try accessing the file in php with something like fopen() or file()
There is an exploit in versions < 0.2.1 that did not process purchases correctly. 0.2.1 was a critical release that fixed that. Now, there are no other exploits that we know of for the current version.