That's the biggest problem. It's not a wall that you suddenly hit. It's like constantly trying to cross a wild river. You always hope it's just three more steps and then you are on the shore. But there's no shore to reach, it keeps moving away from you.
But I'll give you examples. VAT is different and keeps changing. One country would lower VAT on books to 5%, then to 0%, then they have an election and say ebooks for children have the full VAT. Multiply by all countries in EU.
Or you want to send the book to Germany. But no shipping company would take it, because they have introduced a special system where you have to pay a fee and register and report each package's wrapping material. What kind of envelope, did you wrap it in plastic, etc. I'm not joking at all BTW. You really have to do this. Multiply this bullshit by all member countries.
If you knew this in advance, you would never do it in a million years. But since people believe it can't be that bad, they go into the river. And there's no shore...
On the internet especially with Bitcoin, you can just do things...