What is the ExtraProfit and how can I earn it?

Each day, SixthContinent shares a portion of it's profits with all users, this is the credit that you see added to your eWallet every day. 
If you choose not to make a purchase (with or without the use of your credits) within a calendar month, you forfeit those credits and they are distributed back to all of the users who have spent at least $300 in cards during that month. 

So, if you want to earn lots of extra credits each month, aim to spend a minimum of $300.