I was quite amused by Lydia's comment about asking the maid to mend her muslim dress. It seemed so foolishly irrelevent. Then she says she's off to Gretna Green. Is Austen using the letter to reveal Lydia's intentions ? i.e Gretna Green, then back to Brighton as Mrs Wickham. To her mind, Wickham back to milita duties, ballls so she'II need her muslim dress to keep her dance date with Lieutenant Pratt. It's interesting what is not in Lydia's letter is also revealing. She doesn't seem aware Wickham has deserted the Brighton Regiment so cannot return nor is she aware Wickham doesn't intend to marry her. In short, it shows how poor, silly Lydia has no understanding at all of the ruthless man Wickham is. I guess it also reveals Lydia's foolish nature and what an easy dupe she was.