I also wanted to wrap things up as much as possible before my demise & researched several options, including Neptune Society. They were the most expensive for my very simple “disposal” requirements (no casket, simple cremation). I ended up buying an insurance policy (which is how you pay for death related expenses in advance apparently) with a local funeral home. I think it was about $1700 and the Neptune quote was around $3000. Neptune still sends me ads in the mail.
So my only advice is shop around.