I guess other letters from RBS weren't specified to be delivered to the oriiginal adress only.
If they are just sending out general stuff they don't care but they want the bill to go to you directly.
They are saying because they couldn't contact you directly, you have to pay €25 in the extra costs involved to track you down.
Given that they probably had for an individual case contact
Deutsche Post, adjust their system for a new address and then re-issue your bill, €25 is a fair enough rate as far they're concerned.