HTTPS works for appspot domains, so I thought I'd redirect them to bemmu1.appspot.com for the checkout. That wouldn't exactly inspire confidence in the service, but just until App Engine gets around to adding proper support.

Roger that, that's what I thought you might plan on doing. Let's hope they solve the SSL issue soon -- since I'm also a fan of GAE!

Btw, thanks for sharing, it's been quite inspiring to read.