Square is a company built by Jack Dorsey that enables any smart phone or tablet with an internet connection to accept credit cards. This invitation is incredibly simple and clear. The card reader is free and all you need to do to get started is enter your email address and a password. Done.
The fine print is even crystal clear. One flat rate per transaction (2.75%) goes to the company, the rest is yours. Simple, clear, didactic. Terrific overall UX.