How to include material after the letter in newlfm 25 November 2018

TL;DR: If you want to include additional material after the closing of a newlfm letter, forget the builtin \restletter commands. Add \newcommand\closelfm{\closlfm\let\closlfm\relax} to your letrinfo.tex file, put \closelfm where you want to end the letter (optionally followed by a \newpage), then write the additional material using standard L^aT_eXmarkup.

I use the newlfm L^aT_eXpackage for all letters I write, personal and professional. It has some nice advantages over the standard letter document class: you can have a database of different sender and recipient addresses, signatures and letterheads; it's easy to create custom letterheads that include images; you can have the correct letterhead and signature selected automatically based on sender or recipient. And many other features I don't make use of.

I have a love-hate relationship with this package, though. It has some quirks/bugs, especially to do with page layout and spacing. And whilst the documentation is extensive, it's not always the clearest. Whenever I need to do anything slightly different to what I've done before with newlfm, getting it to do what I want typically involves hours of effort and frustration the first time around, and I often ending up resorting to ugly kludges to get the letter written without wasting too much time. (This probably says more about my limitations, than those of the package.) Nonetheless, once you have a letter layout configured just the way you want, writing new letters with the same layout with newlfm is a breeze.

I was recently writing a cover letter to accompany the revisions I was submitting to one of my research papers. I wanted to attach the detailed response to the referees' comments after the letter (which contained a lot of maths so was best written in L^aT_eXtoo). The easy thing would have been to write the cover letter using newlfm, write the review response as a separate L^aT_eXdocument using some other document class, and stitch the two PDFs together using the pdftk command-line tool (or similar). But having to write these as two separate documents bugged me. I was already writing the cover letter in L^aT_eX. Why not just write the detailed referee response in the same L^aT_eXdocument?