the "package file invalid" message is pretty much a generic message when some part of the download from the market fails. I've been going through this with a new Nexus 4. If i tried to get apps via wifi, I'd get the error but using mobile data it worked every time. Over the the past several days it has been clearing itself up, but I can't say it's anything I've done.
Most likely what is happening is that either the wifi drops at the end of the download so the checksum cannot be validated or the connection is dropped during download, corrupting the file. Sometimes if you try repeatedly it will eventually be successful, or sometimes if you install a small app (under 150k) and then go back and install the large app, it will work too.
You can try clearing the play store app cache and data, and forgetting the wifi network and reconnecting. Even rebooting the phone could help, but I really am not sure what's fixed it for me.