Teslarati seems to have discovered the alternative while poking through the code of a MyTesla page for people who have ordered a Model 3. The news outlet found some inactive code that, when changed to be visible, revealed a section in the payment info box that talks about something called "Supercharger Credits" and a blank next to kWh. We are inclined to agree with Teslarati that this box probably allows users to purchase amounts of power ahead of time to spend at Supercharger stations, and that the box also displays how much power is left in the account.
This also answers one of the lingering questions about how Model 3 owners would pay for powering up at Supercharger stations. Elon Musk made it clear not too long ago that Model 3 owners would only have free, unlimited access to the network of chargers by purchasing an option package. However, he didn't say what the alternative would be for people who didn't buy the package. While this online payment system is likely a key component of the Model 3 payment strategy, it still leaves open the question of whether any other options will be available. For instance, will some stations have credit card readers if owners are out of credits and don't want to fumble on their phones to buy more?
As for bringing in revenue from Superchargers, the Model 3 strategy is just a start. Even though the Model 3 is expected to significantly outsell Tesla's other models, there will still be significant number of Model S and X owners charging for free, costing the company money. One idea that has been floated is a variation on a fuel station classic, the car wash. Of course these car washes would be environmentally friendly and cost far more. No decisions seem to have been made regarding this idea though. We'll also keep an eye on the Model 3 payment system for any updates.