For Boeing and other aircraft manufacturers, the practice of charging to upgrade a standard plane can be lucrative. Top airlines around the world must pay handsomely to have the jets they order fitted with customized add-ons.

Sometimes these optional features involve aesthetics or comfort, like premium seating, fancy lighting or extra bathrooms. But other features involve communication, navigation or safety systems, and are more fundamental to the plane's operations.

Many airlines, especially low-cost carriers like Indonesia's Lion Air, have opted not to buy them — and regulators don't require them. -New York Times