You've probably already found this document. Make sure you start here: https://meraki.cisco.com/lib/pdf/meraki_whitepaper_captive_portal.pdf
Look at the "EXCAP Overview - Sign-on Splash" section, which is pretty close to your use case.
You can use the Walled Garden settings on your SSID to allow the user to interact with your site (to login and use their credits) while not providing access to everything else.
You will need a back-end application to serve the captive portal pages, handle login, do the points exchange, terminate the session when the time limit is reached, and confirm authentication via RADIUS.