by custom

See: pro forma
References in classic literature ?
It is true, that what is settled by custom, though it be not good, yet at least it is fit; and those things which have long gone together, are, as it were, confederate within themselves; whereas new things piece not so well; but though they help by their utility, yet they trouble by their inconformity.
Color concentrate pellets are supplied for nearly all thermoplastics, either from standard colors in stock or by custom color matching.
Celebrated by all the members of WCO, International Customs Day is a wonderful reminder of important role played by customs bodies in the world.
Chicago attorney Edward Fox says he represents about 90 African American women--most of whom went through O'Hare--who allege their civil rights were violated by customs agents.