Do y'know, I've made the simple error of just plain not reading
I was typing my credit card number in 'your name' and and my name in 'credit card number'. Doh.
This has worked on all of the PDB books I bought in fictionwise to use on my ipod touch, which I'm now converting to use on my E-Reader PRS 600. I'm pleased I can go ahead and read these in a more comfortable way now!