The travel deal was so good that I didn't think twice about booking it immediately.

didn't 是 do not 的缩写形式，表示否定。 think 的意思是"思考、考虑"。 twice 的意思是"两次"，think twice 字面意思是"思考两次"，引申为"犹豫、再三考虑"。 didn't think twice 这个短语表示"毫不犹豫、不假思索"。