BookingCenter places a BCC email to all emailed LETTERS sent from BookingCenter if you have chosen to set a 'CC' Booking Confirmations' in the Setup | Settings | Contact Information area.

If you do not wish to receive either the BCC email when emailing LETTERS or receiving the 'CC' Booking Confirmations' email, then delete the email address in the location above.

There is no way to get a 'CC Booking Confirmation' and not have the Letter auto-populate the BCC email in a LETTER. However, prior to sending a Letter via email, you can simply delete the BCC email field.