Nah! Take them on. The principle of trying to rip you off. Even if you don't win, worst case, at least you took on the fight than thinking "what if".It's just not worth the trouble and stress it would cause the Customer could challenge it saying anything they weren't cleaned, weren't cleaned to a sufficient standard, disputes charge whatever? Get on with life move on at least they can't ask you again.
If they would rely on disputing anything argue that they could have contacted you before it got this far.
Put it this way, if you were to be arrested for something you didn't do, would you keep quiet until it went to court and rely on the Magistrates opinion or would you scream your innocence at every opportunity. A guilty man would know that he had committed the crime and would have less passion to plead his innocence.
Bit extreme but you know what I mean.