Sheer volume and profit allows them to subsidise this.
In other countries this business model hasn't worked as lots of people order too much (every possible size that might fit them) with the plan of sending most of it back, or worse, buying, wearing for a night out, and then sending back.
My last order came in 5 seperate packets, one from serbia and some from Germany. I sent the same number of packets back. You have to wonder but this is their business model.
In Switzerland if you buy a packet of teabags online it'll cost you more than 10CHF for the postage even though there's a 200% mark up on the original price so it's no wonder people use amazon or zalando etc. Generally there the postage (and possible return) is built into the profit margin.
That's insane. I think I return about one in fifteen orders on Zalando and one in a hundred on Amazon - we order a lot from Amazon. Sometimes Amazon don't even want the item back as they can't sell it if the reason for the return was 'damaged in transit'.
It's also worth checking out the prices on the Zalando sites of neighbouring countries as often the Swiss site has a huge unjustified mark-up. It pays to shop around (obviously more so if you are near a border or visit one regularly).