With versions 3.1, 3.1.1 and 3.1.2 the secure URLs produce 404 pages. For example: https://wpannex.com/payments/base-website-thumbs/ gives a 404 with those versions.
If I turn off the "Secure Post" option then the URL resolves correctly and shows my payment form (of course the payment form won't work because it automatically disables itself if the connection isn't secure.) For example: http://wpannex.com/payments/base-website-thumbs/ (same url with http instead of https) doesn't give a 404 error.
However when I roll back to the 3.0.4 version of the plugin https://wpannex.com/payments/base-website-thumbs/ works just fine.
As we talked about in another thread, this seems to have something to do with the page being a child page. As a test I made it a top level page and https://wpannex.com/base-website-thumbs/ worked with version 3.1.2 of the plugin.
Unfortunately in my situation I need the page to be a child page so I reverted back to version 3.0.4 of the plugin which is working for me at this point.
Hopefully that helps you trouble shoot some. I really do appreciate all the work you have done on this plugin.