I've never seen a web application which does not use the subscription model, so I don't know if a pay once model will work since the costs of running C3 on a server and maintaining it must be quite high.
Although I think that the best way to do so is by making the payment model pay-once and charging extra per each exporter or feature. In short, the only other option besides the subscription model is the pay-once/pay-extra-per-package model:
- Construct 3 lifetime registration = xxx $
- Android exporter package = xxx $
- iOS exporter package = xxx $
- NWJS/Electron exporter package = xxx $
- Marketing boost package = xxx $
etc ...
Even so, I don't think that it will cover their monthly bills.