At the end of Chapter 35, after Mrs. Joe has died, and after Biddy has told Pip lovingly "how Joe never complained of anything...but ever did his duty in his way of life, with a strong hand, a quiet tongue, and a gentle heart", we see Joe "already at work with a glow of health and strength upon his face that made it show as if the bright sun of the life in store for him were shining on it". We know good things are coming in Joe's life, and it wouldn't be surprising if they concerned Biddy, who had such genuine concern for him even then.
A few pages into Chapter 57, when Pip first revives enough to converse with Joe, Joe tells how he learned of Pip's illness through a letter the postman brought Biddy, and characteristically goes off on a tangent about the postman "formerly single he is now married though underpaid...but wealth were not an object on his part, and marriage were the great wish of his hart". It is arguably not the postman, but Joe himself of whom he is speaking. Underappreciated by Pip and unconcerned by wealth, his marriage to Biddy will indeed be revealed one chapter later.