I know you bought the autotopoff.com already, but I would put a fail safe on it as a precaution, like a timer with a second solenoid or something to minimise risk.
Bought mine brand new from them directly with double float, within 2 weeks (literally out of the box) the solenoid failed, flooded my basement and caused a ton of damage to walls, floors, etc.
Plus didn't have a good experience with service, they didn't want to return the unit for my money back, just sent me instruction after instruction.
Not a good experience here at all.