I have to agree L4M4. Arrow must have made a special contract with his shipping services, or sort of. I prefer not to abuse this, hope it will last. O:-)
For very small orders a standard postal package is the best way, hope they can realize if the above cannot hold.
Well - it was fun while it lasted
Otherwise, handling is now $12.99
So, the answer to my question, how they did that with DHL Express from the US to Europe for 2$ items is: They din’t. They made a massive loss on those orders.
But even for 50$, free DHL express shipping is still a bargain. Too bad, that I very rarely need so many LEDs 
What about customs fees? 
Nope, for EU customers they offer DDP - regardless of where the parts are stocked - and as far as I can see it’s by default, no extra cost.