The term 'BILL ADD-ON' refers to additional charges that may be added to a bill. These can include a TIP (TIP), a TAX (TAX), a RIDER (RIDER), or a sales TAX (SALESTAX). Tips are usually voluntary payments given to service providers, such as waiters or taxi drivers, for good service. Taxes are compulsory payments made to the government, and are used to fund public services. Riders are amendments to a bill that add extra clauses or conditions. Sales taxes are taxes imposed on the sale of goods and services.