jimi, I know of some other issues related to the order total preview section. I'm planning on correcting them for Alpha 8 (maybe not this 7c release), but this one is unexpected. Can you look in the source for that page for the following:

It should be a bit of Javascript to add the subtotal to the order total preview.

I don't know why that wouldn't be getting added. It's working fine on my test site. Is this a regular thing, or only every once in a while?

(Or if you have Firebug, does anything of interest show up in the logs?)