Supporter blogs not being upgraded

I have supporter installed, and I'm trying to test out supporter only features on a couple blogs. However, for the life of me, I cannot get a single blog to upgrade to supporter.

I've tried manually extending the blog from the supporter admin panel. I've got the paypal sandbox set up, and I go through the "purchase" process with the sandbox, but my blogs just are not upgrading at all!

I've checked the notify_url and it's pointing to the mydomain.com/supporter-paypal.php file.

My supporter-paypal.php file shows "Error: Missing POST variables. Identification is not possible." when I visit it directly.

Once I go through the upgrade process, or if I extend a blog, it shows the paypal subscribe buttons still.

If I go to the "disable ads" menu on a blog I try to upgrade, it shows:
Maximum blogs: 10
Currently disabling ads on: 0
Remaining: 10

I'm at a loss of where to keep troubleshooting. Any ideas?