E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Salt an Invoice (To)
is to put the extreme value upon each article, and even something more, to give it piquancy and raise its market value, according to the maxim, sal sapit omnia. The French have the same expression: as Vendre bien salé (to sell very dear); Il me la bien salé (He charged me an exorbitant price); and generally saler is to pigeon one.