I understand wanting to take some of the pressure off heirs during a stressful time for them.
I purchased a prepaid cremation from
Baldwin Brothers and feel like it was a good deal. Did it all online. Mine was straightforward and inexpensive ($650) but they offer a full range of options. And if there's a problem I won't know about it. :-)
As others have said, the important thing is to make your wishes known to your executor(s) and, if necessary, make sure funds will be available to them for immediate use.