I have to admit I'm a bit stumped as to what you're referring to when you say "free pre-order".
Do you mean a free book which you pre-ordered (in the sense it hasn't been published yet)?
Otherwise, anyone can pre-order a book, without an additional fee to the actual book price. Evidently before you pre-order or order you need to have set up an account with amazon where you do have to specify a credit card number that will help pay for the book.
And yep, you can cancel the pre-order by going to "Digital orders" on your account.
Hope this helps
EDIT: right, so I'm a slow typer 