This is a sleazy marketing technique.
I have purchased your services as gift certificates for the past 3 years. I am annoyed and disappointed that when I have specified my email in the past --- you make me look for and uncheck the email preference question at each purchase. This is a sleazy marketing technique.
You are turning a convenience into an annoyance which will stop future purchases.
Tell your webmaster to set the preference defaults to the client's previous answers--if they want it, they will change it.