I take it you cant do it through the "upgrade my laptop" section?
EDIT: For some reason when I read that back to myself I sound condescending... I don't mean it like that! (smiley's always help!)
Also (and I figure you probably know this already), might be quicker to ring PCS Sales if you can (or email if not maybe?) - You can purchase one straight away if they say yes then. Of course, PCS does seem to be commenting on the forums very regularly at the minute