The term 'PURCHASE ADD-ON' can be broken down into three crossword solutions: TAX (3 letters), SALESTAX (8 letters), and STATETAX (8 letters). TAX is a general term for money that is paid to the government, usually based on income or the cost of goods and services. SALESTAX is a type of TAX that is charged when goods or services are purchased. It is usually a percentage of the total cost of the item. STATETAX is a type of TAX that is imposed by a state or local government. It is usually based on the value of the goods or services purchased.