The lighthouse would benefit all sorts of shipping besides the mail service, so
I hear a symbolic overtone here, in which "mail service" stands for
all human communication. The lighthouse on the Great Isabel symbolizes the individual's
attempt to communicate the truth of the self to others across the void of subjective
isolation -- one might indeed say that it functions to improve "mail."
Considering that so far in the novel, communiques of every sort have entailed
errors, deceptions or unintended consequences, from Mr Gould Sr's correspondence
to Antonia's gun-running missive to Charles Gould's "letters of fire"
appeal to Holroyd, some sort of improvement in this area is clearly wanted.