I don't think strictly speaking The Woman in White is epistolary, ie written in letters. It is a series of memoirs by different narrators, collected after the events.
Evelina by Fanny Burney is in letters (I haven't read it) as is Jane Austen's early Lady Susan.
I thought Humphrey Clinker was sweet and I'm sorry I'd forgotten it. As I remember, the letters are from a wider range of personalities than in Pamela or Clarissa - for all its length, there are only four significant correspondents in Clarissa - Clarissa, Lovelace and their confidantes.
If those memoirs were shipped over the mail then they were in fact, and law, and in theory "epistolary". LOL!!!
There has never been a single, great revolution in history without civil war. --- Vladimir Lenin
There are decades when nothing happens and then there are weeks when decades happen. --- Vladimir Lenin