Quick as a flash after my last post about online ordering systems, Hadley Rich suggested shopify, which while a hosted system, seems to be written by people with a clue. Railsy kinda clues, which means the designs are lovely, and they've focussed on doing one thing well.
Anyway, the nice thing about the shopify service is it's got a templating system you can both hack and understand nice and easily. So even though all these systems are set up for multiple products, I should be able to steer it into being a single-product system. What's more, the really cool stuff I actually want to play with, Google Website Optimizer, will be very easy to integrate.
Now to decide if the tax bollocks is worth the effort. The ATO material seems to suggest you don't have to charge GST if your turnover is, or is expected to be, under $75,000 per year. That would simplify things greatly, but they don't actually come out and say that. Guess for such things you need expert advice.