Yes. It sounds very broken but people usually understand anyway. There can be misunderstandings with plural/singular. Since brev is an ett word, brevet means 'the letter' and breven means 'the letters'. Of course people are likely to misunderstand you if you use the wrong form in cases like this.
bokstav = alphabetical character (bok = book, stav = rod, in this case it hails from the vertical rods people used to write runes in stone! :) and the verb "att stava" = "to spell" is also related)
brev = mail/post (etymology: a little connected with english "brief" - short message)
The machine tries to match your input to the closest accepted answer. If you pick the wrong article and type "en brev", it will unfortunately think that the closest answer is en bokstav rather than ett brev. At least this is how it works at the moment.
If you really did type ett brev and didn't have that accepted, there must have been some bug/glitch.