I can't speak for proper etiquette in the US, but in the UK, single women seem to be called Miss (on post and in formal address, like at the bank, etc.) until they are married, and then they're called Mrs - Ms, which I preferred as a single woman in my 20s and still prefer now, especially in professional correspondence, seems to be reserved for divorced women. I hated being referred to as Miss MaidenLastname before I married, and I hate being referred to as Mrs MarriedLastname now that I am married. I love my husband, but I am more than just his appendage!
If it were me, and I were paying for my own wedding (as DH and I did), I would address the envelopes exactly how I felt comfortable - whether that was using Ms or otherwise. We had over 250 people on our guest list (our wedding was much smaller - about 95 - but there were a LOT of family and friends who expected invites even though we knew they wouldn't be attending) and we omitted forms of address altogether and just used Firstname and Firstname Lastname (or Firstname Lastname & Firstname Lastname in case of couples with different lastnames) on our invites, and as far as I know no-one (even the proper Southern ladies) was offended!
It really all just depends, like so many other aspects of wedding planning, on how much you care about sticking to formal etiquette.