I did 40 credits of post-bac informally and didn't get a letter from that (about 4 different schools!). I did get two letters from my grad work. I didn't get a letter from my old UG work because it was too old (nobody there would even remember me, and I wasn't pre-med-- heck, all those profs are probably passed away by now anyway!!).
I also did get letters from my work and volunteer stuff. Since work has been my life for 10 years, and it's relevant, it worked out really well for me.
The advice from my advisor was:
1. coursework letters are required (most schools specify them coming from profs who taught you in a formal course) but the grad coursework was the most recent and would carry the most weight in my case
2. significant work experience should provide a letter from a supervisor. In my case it was relevant to medicine but even if it wasn't, they could say something about your personality and work ethic and if they would want YOU to be their doctor and why... blah blah blah
3. significant volunteer experience or shadowing letters are VERY important
I ended up having a lot of letters, but as a non-trad it was really necessary to really showcase all that I have done and am now. I think it was 2 from science profs who taught me, 3 from work, 2 from physicians I shadowed, and 1 from volunteer stuff I did (plus when I applied DO I added another shadowing letter from a D.O.). Yep, ridiculous excess, but this is your time to shine.
Get your BEST letters (quality is more important than quantity). In the case of non-trads who have been out of school for a while, the committee letter or letters from profs might not be that great and glowing unless they REALLY remember you well. So, dig down to what you do now and find people who can write strong letters for you.
If you ever need advice... I'm here for the PMing. 🙂