결제하다 = complete a transaction. Formal term used at financial institutions or larger businesses. 계산하다 = 1) compute / calculate, 2) complete a transaction (used informally at small stores and restaurants). 결제하다 / 결제되다 is mostly used at large institution where the transaction is fully recorded and a formal receipt or some other proof is issued. 결제되다 is a common term used by clerks at large businesses (2만2천 원 결제되었습니다). 계산하다 is an informal everyday term used in the sense of paying for something with cash, credit card, or some other way (친구와 저녁을 먹은 뒤 내가 계산을 했다 = After a dinner with my friend, I took care of the bill).