My experience with filing via the AOPA flight planner (which I believe
uses DUATS as its backend) is that it does indeed reject PA-28, PA28,
etc, and only accepts P28A, as you said and the FAA pub say. My CFII at
the time when I discovered this told me I was crazy.

Interesting that the designators for lots of other PA-nn were not
changed around.

-- dave j