The customer almost always wins those. And the merchant always has to pay a fee for the chargeback, even when they win, so they're incentivized to avoid them.
The merchant agreement isn't as effective as a well enforced law, but it's pretty close.
I would be fine with waiving my right to returns but this is not possible on purpose, so my only options are to shop somewhere else (often not possible) or found a company (not possible because it would be Liebhaberei - "Running a company without intent to make profits").