You know how they say never to assume?
I just received an email from Debbie answering this question. She says Axiom only charges you the difference up front.
So much for my logic and assumptions!
Now I have to figure out my order as I payed via PayPal with no trade-in information - meaning I paid the full amount up front - oh well, I know I'll get it sorted with Axiom, they always look after their customers.
Apologies for the misleading assumptions.