If it really was just "drizzle" I struggle to see how the contents of a bag could be "soaked". Was there a long delay in receiving your bag for some reason? Most modern, good quality bags are pretty waterproof these days. So I struggle to see how this could happen. I would be talking to the manufacturer of the luggage rather than the airline.
If I received a claim for €40 in such circumstances I would probably reject this. I don't see that blame can be clearly allocated here. Incidentally, could the OP not simply hang them up to dry, or if urgent use the hotel hair-dryer and iron to press them dry? Why was it necessary to pay €8 a shirt to get this done?