[Wowfunhappy]

The first problem that comes to my mind--clients will remember both wifi networks and may continue to choose the open network when e.g. waking from sleep.

The user can go in and forget the open network of course, but most won't know to do that.

[stereo]

It would theoretically be possible to bounce clients which you know are already authorised on the paid network.

[gorlilla]

QR code to connect to the _open_ but _hidden_ SSID. Instructs user to join WPA-EAP with supplied credentials once they've paid. Remains available to connect via QR in case customer somehow misplaced creds but avoids auto-reconnect during scan.

[lxgr]

You could just prevent access to the wider internet on the open version!

[Wowfunhappy]

Yes but the client will still connect and the user will wonder why their internet isn't working.

[IggleSniggle]

Use Bluetooth instead of WiFi for the password generation channel