Absolute acceptance required: acceptance must be unconditional or comply with a prescribed manner to form a contract. An offer becomes a promise only when acceptance must be absolute and unqualified. Acceptance should be expressed in a usual and reasonable manner unless the proposer prescribes a manner; if a prescribed manner is not followed, the proposer may, within a reasonable time after communication of the acceptance, require acceptance in the prescribed manner, but failure to do so constitutes acceptance of the communicated acceptance.