The letter Arya finds is the one Sansa sent Robb. Jon recalls the wight which was brought to Castle Black.
Arya shows Sansa the letter she wrote to Robb, accusing her of collaborating with the Lannisters.
Tyrion reminds Bronn his offer to outbid whatever fee someone else pays him. Littlefinger mentions the letter Cersei forced Sansa to write to Robb.