> If you just have to accept courts will void 1% of your transactions (costing another 2% in legal fees) then you just make everything 5% more expensive to cover it.

That's an absurd exaggeration in regard to the issue at hand. Almost certainly far less than 1% of purchases by minors are voided, and NONE of those involve legal fees unless the seller chooses to go to court rather than refund.

In fact, I'd be willing to bet money that there are overall far less purchases refunded in Germany than in the USA.