Found the answer myself: Galaxus sent me to another supplier's offer that only sells in twin packs. I had to switch to the Galaxus offer via "other suppliers", but it has "unknown delivery date". Then I got a reasonable price and single sale.

This is called graduated pricing (quantity discount). The more you buy, the cheaper the price per item. You can also order just 1 piece.

You don't have to. You can select the number at the bottom. I tap on : via a link to the product where two were active.