COMS 17th edition
November 7, 2019
Your citation format for Letter is not correct per COMS 17th edition. There should also be an option for letter in a published collection. See sec. 14.111. Please update.
November 8, 2019
Given that you can cite multiple letters from the same book but CMoS lists only the book in the bibliography (and Zotero can't do that in principle), I'd just treat these as books and then add the specific letter as a prefix. This isn't 100% -- it'll include the author name twice -- but it's probably the best that's reasonably possible
November 11, 2019
Thanks. I treated it as a selection in a book, but ended up adding it manually.