bonus vs sweetener

bonus

noun
  • Something extra that is good; an added benefit. 

  • An extra sum given as a premium, e.g. to an employee or to a shareholder. 

  • An addition to the player's score based on performance, e.g. for time remaining. 

  • One or more free throws awarded to a team when the opposing team has accumulated enough fouls. 

verb
  • To pay a bonus, premium 

sweetener

noun
  • Something given or added to added to a deal to sweeten another's attitude, especially a bribe or kickback. 

  • A shill bidder at an auction. 

  • Something added to food to sweeten its taste, especially an artificial substitute for sugar. 

How often have the words bonus and sweetener occurred in a corpus of books? (source: Google Ngram Viewer )