We developed a payment system through Skype that allows a pay-per-minute video/voice call conversation in real time. Technically we made a plugin for Skype, that the service provider (let's say a teacher) installs next to Skype and can ask a customer (let's say a student) to (1) make a Minutizer.com account, (2) add funds via credit card/bitcoin/etc. and (3) just call him as usual on Skype.