I don't think that this is about charging for a signature; it is about the time, experience and effort required to ensure that the signature is appropriately appended to a document which is in fact, correct.
It's not always as simple as just sign in the correct box, especially for a stranger.
If an authorised signer fails to check every detail, he or she may well end up unable to sign anything else, and it may even prevent other more relevant things being done.
I would hope that once the CAA find that you have effectively falsified a document, you can forget about them allowing you to keep on doing it. Simple mistakes happen, but that's what the signature of an experienced and licensed person is supposed to pick up.