Nonfunctional slack-fill is empty space in packaging that serves no purpose. Potato chips, for example, have functional slack-fill in their packaging because the empty air in a potato chip bag protects the chips from pulverization. Nonfunctional slack-fill is when a seller deliberately deceives the consumer with excessive packaging. As the Russian idiom goes, “all feathers, no fluff.”
Or, in academese, synonyms include: padding, fluff, filler, bullshit, and–at its worst–rhetoric.