@vbabka @ljs apparently i'm bad at interpreting numbers and bad at web forms. Here's the final truth ;-)
The bugs:
1. I forgot to add the second coupon. So we never know the truth.
2. PREZDAY25 was discounted from the full order not from the most expensive book.
Well.. Umh... Hmm... lol.
Well, in the end of the day this did not go so badly. I still got -30% instead -25%.