You're absolutely correct--
Written by Kalyn
(9/24/2005 8:48 p.m.)
in consequence of the missive, Love Letters?, penned by Chris DS
they could only exchange letters if they were engaged. In fact, in Sense and Sensibility, if I remember correctly, Elinor believes there must be some sort of understanding between Marianne and Willoughby because Marianne does write him when she gets to London.