The cost of each day pass is $9.99 USD, which gives you 24 hours of unlimited 4G data for up to 10 devices.
When you login in to your myGeeFi portal, you can buy as many day passes as necessary. Each time you activate a new day pass on your GeeFi device, you'll get 24 consecutive hours of Internet access.
There is no need for you to worry about stopping the service, or automatic/continuous billing. GeeFi will never automatically activate a day pass. Each time a day pass expires, GeeFi will explicitly prompt you to determine if you want to activate another day pass.