When you add a contact person to your trip, you spend one credit.
Buying credits keeps us in business so that we can be there when you need a message sent. The good news is that the app itself is free, so you can sign on from whatever android (and soon, iOS) device you have handy. Your credits follow you, not the device, so put the app everywhere you need it.
Oh, and in case you were wondering, Dan and Chuck are real people, not a clever marketing ploy. We built this because we believe in this idea. And Dan has the scraped knees and biking stories to prove it.