Thanks for pinging me @ScruffR
It should be the case that all associated devices that are added will incur charges past 100 devices, so you are correct that adding your unsold devices would incur additional charges each month. Therefore, I can see why you would want to avoid this.
With that said, we do have libraries available to handle new device setup. You could consider building an app for your user to setup the device through their mobile phone, making using of our provided Android or iOS SDKs. This would not require them to use the CLI for setup purposes and also opens the doors to simple-auth and other functionality that would allow you fleet style management as the device owners, but creating a white-labeled experience where your customer is working through your ecosystem, rather than Particle’s directly.
If you went with a mobile application route that handles the setup of Particle through the SDKs, the devices could still be unclaimed in this state and it would allow you to prevent premature billing for ownership of the devices.