When you pre-order a reMarkable you will receive a confirmation email which will serve as the receipt. It will be sent to the email address used when placing the order.
If you haven't received the confirmation e-mail, then first check your spam folder in your e-mail client. If you still can't find the confirmation then submit a request to us by using the form on our help pages. When submitting the request please let us know about the date of the pre-order, your name and your shipping address.