How interesting. Makes me want to set up a similar service where I am. $500 minimum, mark it up a steady 10% based on price when the order is placed, and offer to buy the coffee when you meet at the local cafe to make the swap.
Hard to make a living unless you're doing it full time, but the hobby would pay for itself and then some, and you'd meet some interesting people and learn your way around the ecosystem.
As for the money... keep the cash! Use the cash! Cash is convenient, cash is universal! I've been thinking about going all cash for anything but purchases that must be made online, and even those can be made with prepay cards that can be bought with cash. edit: a moment of thought reveals you have to put the cash back into the system somehow or another in order to fund more bitcoins. So, there's that fantasy shot.