Dont buy too many because studies show most people wi forget where they put it or dont remember receiving one. Use vistaprint and find a code on retailmenot but only give them in person to people you meet but better these days to use an e-card. You can downloaD free apps to make an e-card or once you buy from vista it turns it into an ecard.
If you are service business one tip is to look up people that just bought a home and send them a postcard instead. They are on a high right now so they will buy anything.