The Fox didn't want this extra complication

Xiao Ziyi climbed up the dais with some awkwardness to show them the letter. "I haven't read it yet," she told Xiao Zai, handing him the folded paper.

Xiao Zai read the first few sentences with a deep frown. "It's from the Third Princess."

Unable to hold back his curiosity, Chu Yun peered over his shoulder, and read aloud, "'I've hurried to send this letter before our departure, for fear that I won't be able to say this when we meet in person: don't trust my mother."

The calligraphy was sloppy and rushed, matching someone trying to send out a letter undetected. It was stamped in ink with what looked like a peony pattern, similar to that of the Su crest. Chu Yun thought it looked like something that might have been carved in a piece of jewellery. 

Something the Third Princess would be wearing when they met, if she was determined to attest to the legitimacy of the letter.