To me the main concern is that this shouldn't even be a code change: this stuff should be a matter of configuration. In a well-written system the dev time for it should be zero.

But in general, the whole letter feels wrong: either the complications of such a trivial change depend entirely on legacy code that isn't responsibility of the current maintainer, and therefore the client knows well how hard and expensive even simple changes are (and this should have been made clear when the project started); or the maintainer is asking the client to pay for developments (refactorings, tests, or a different, more general implementation of the feature) that weren't agreed beforehand.