Americans who are receiving a year-end bonus for a job well done may be sorely disappointed when they open their envelope to find a big chunk missing. Up to a third of a cash bonus can get swallowed up by the IRS’ special tax withholding on cash bonuses, or what it calls “supplemental income,” on top of Medicare, Social Security and state taxes. The federal flat rate for bonus pay is 22% for supplemental income under $1 million. Add Social Security (6.2%), Medicare (1.45%), and state taxes, and total withh ...