Please provide at least one e-mail address that you check frequently. This will be your primary e-mail address for any notifications we might need to send you in the future, including the links we will send you to download the documents you buy. You will also use this e-mail address to sign in.
If possible, please provide a secondary e-mail address as well. We strongly advice you to do so. If you do, we will send all the notifications to both addresses. Doing so we will reduce the problems derived from e-mails not noticed, invalid e-mail address, e-mails filtered by spam solutions, etc.