It would have not cost anyone more ink or paper to make this 1$ bill into a 100$ bill, and I would have been happier if that were the case, oh why is everyone in the world so against me?
In other words, the process that leads to the production of a good or service is not what you pay for, what you pay for is the end product: if you pay for a battery with a capacity of 40kWh, that's what you get, and if you have the option of increasing its capacity later on, well even better! Or, you can get the extra capacity from the beginning, perhaps with a small discount on top of it. Proof of this is that, if they didn't tell you (or you hacked the system), you wouldn't even know about the possibility of the increased capacity. Then if you find out, you feel entitled to that extra capacity, just because "it's there" and part of the thing you bought.
