So I got this extra card from someone and sent them an email through deck box about it. The guy seemed very active during the trade, so I'm surprised I havent heard back from him yet...... Then I realized that I have no idea how the email system works on deck box. help? (I log in through facebook, if that makes a difference)

It sends an email to the address associated with their account. It includes your message and some extra text saying that it's from Deckbox. You can also try messaging the trader in the chat of the relevant trade.