Informally known as tipping, in the United States tipping is voluntary.
Tips are typically rewarded for services performed, as well as to supplement an employee's income (gratitude).
Tips are at your discretion for above and beyond services provided to you.
Recommendations for housekeeping - $2.00-3.00 per day, Bellman - $2.00 per bag.
(Disclosure: all tips or gratuities not outlined in the contract are discretionary)