From the Booken FAQ:
How do I cancel an order?
To cancel your order, send an email as soon as possible to
shop@bookeen.com indicating your order number. Re-stocking or administration fees might apply in certain cases. In case your order has already been shipped, you may return it within 7 days upon arrival by getting in touch with our shop team and requesting a return. See How to Return.
(I have tried this and it worked out nicely. The reason for cancelling was that I found out it would be cheaper (due to VAT) and quicker to buy from a local reseller who had (and still has) the machine in stock)