I had a similar problem, although I din't get a message saying "Package file was invalid" my downloads just kept on getting stuck. So when ever I tried to download ANYTHING off the play store it just stayed looking as if it was starting the download yet nothing downloaded.
What fixed it for me was just restoring the phone to factory setting. For some reason I don't know why but updating from Gingerbread while keeping all my files, settings etc din't work until I did a factory reset. Give that a shot, just of course as you know back up all your contacts and files before restoring.