I suppose that the word "tip" also means a "small fee for service employees" like a waiter or delivery guy. And is it mandatory in the United States for the customers to pay a tip?
Answer from a Native speaker
Yes. Another definition of a "tip" is a small payment towards service employees such as waiters and delivery drivers. Tipping is not mandatory, but it is a custom in the United States. It will be considered very rude if you do not tip your service employees. Usually, you should tip about 20% of your bill. This is because some restaurants do not pay their waiters and waitresses a high enough wage to live, and it allows for employers to pay their wait staff less.