When they say "ONE LOR" they are likely talking about one from a pre-professional committee at your undergrad.
For me, I needed 2 or 3 letters from science professors. They sent the letters to the committee and they composed a "cover letter" either recomending me or not. They also sent the LORs from the professors as well as any others i may have had in one package. The sent them directly to the schools i applied to.
I did not use the ADSAS LOR service. I was told that schools prefer a committee letter. However if you have been out of school for a while, or maybe your school does not have a committee then adsas is fine.