The OP is not the first to complain of what appears to be a minimum access time charge by some Skype WiFi providers. It should be noted that Skype WiFi is apparently acting as a billing agent. They don't operate the WiFi service, and individual providers can have their own terms of service which is supposed to be provided before a connection can be made. See Skype terms of service:
19.4 Skype WiFi
Skype WiFi is provided by one of Skype’s WiFi partners. Your use of Skype WiFi is subject to that partner’s terms of service. Before you start a Skype WiFi session at a specific WiFi hotspot, you will be notified of the identity of the WiFi provider and their terms of service. You will have to read their terms of service, which you agree to be subject to by proceeding with the Skype WiFi session. You can pay for Skype WiFi using your Skype Credit balance, and your use of Skype Credit is subject to these Terms of Service.
If it was Tengo, than I would contact them and check to see if they have a minimum connection time for use of their paid service, or to dispute any charges.