He escrito a Amazon.de y muy amablemente me han respondido "in english" con la solución. Menos mal que me han anulado ellos el pedido, porque sigo sin dar con la solución aún siguiendo estos pasos.
De todas formas pego aquí el correo por si a alguien le puede servir de ayuda.
Un saludo
Please note that you may cancel any items yourself via the "Mein Konto" link located at the top of any page, provided that they have not entered the dispatch process.
For your future reference, I've listed step by step instructions on how to cancel any undispatched orders or items below:
http://www.amazon.de/mein-konto/
After clicking on "Mein Konto" click on the link "Artikel oder Bestellungen löschen".
On the next page, enter your e-mail address in the field "Meine E-Mail-Adresse ist" and enter your password in the field "Ja, ich habe ein Passwort:" Then click on the button "Jetzt anmelden".
Now you will be able to view your orders. Next to each order you will find the button "Bestellung ansehen oder ändern" (= View and Change Order) next to each order. After you have pressed the one next to the order you have wished to cancel you will see a button "Artikel stornieren?" (= Cancel item?) at the same place as the button before.
The next page will display each unshipped item listed with a check box under the "Stornierung überprüfen" column. (Please note that items listed as "1 wird demnächst versandt" cannot be
cancelled, asthey have already entered the shipping process.)
Click the check box next to "1 Artikel löschen" for any items you'd like to cancel. Then click the "Markierte Artikel stornieren" button. That is it. You will receive an e-mail confirmation of your cancellation (in German). Cancelled orders will no longer appear in your order history.