When I try to download the 3.05 upgrade I get the message:

"The compressed (zipped) folder is invalid or corrupted"

That means that the download is invalid or corrupted.

Have you tried repeating the download?

I've tried the download several times since it came out. I get the same message each time.

Try clearing your browser cache and deleting the failed file.
Then download the file again.

Turn off (temporary) any firewalls and/or Antivirus programs you might have running.
Dont use a DL manager.
This is all i can think of

Have you ticked the box next to:

I have read understood and agree to the licence agreement

As not doing this results in the same error as you're experiencing

If none of the above works, PM me your email address and I'll send it to you!

I believe all downloads have to come from cubecart or thru fantastico

Okay