I do shows on snapchat. You charge just the same as you'd for skype: prepaid shows, the client pick how much time he wants and pays for it before you start.
You use webcam sites to get the money. You can ask them to pay through the site you already webcam for or you make an account at adult processors such as IndieBill or Unblur that protects you from chargebacks and gives you a 85% commission (the best in the industry right now).
Be careful to not use paypal, because not only you can have your account banned for life, but they can also get their money back pretty easy with a chargeback.
I usually do shows on snapchat with guys that come from snapchat. If you are getting your traffic elsewhere, it will be easier to use Skype (or other computer-based app) because not many people use snapchat nowadays and it doesn't work in computers. The quality of the image is very inferior on a cell phone than on a computer.
