I do not understand, someone is trying to sell you an inline Dehumidifier for the humidity in the summertime?
Hmmmm, the perfect scam.....
Just so you know, an a/c unit first removes all the "humidity" in the air even before it can think about cooloing the air. To keep it simple, It removes what we call "latent" heat trapped in the water vapor in the air of the home first.
So,.... you already have a dehumidifier.