first thing is who said they were warped? and second what is actually happening to make you think they are warped?
so.... answer the first 2 questions to your satisfaction. then have the service guy (or yourself) spin it on it's axle to see the rotor roll. if you want you could even use a dial gauge and measure the hi and lo's of the rotor.
a few places to look might be the wheel bearing. did it get installed properly too tight or too loose?? did you catch the wheel a bit wrong and not have it sitting true?? so far we've only discussed the symptoms. we need to find the disease.
i got a set of front rotors once and they went south in about 1000 miles. the cause was hard spots on the faces of the rotors. bad castings due to improper cooling or their load was not at a proper poor temp or it sat too long in the pot.
the type of driving you describe makes me believe that the rotors couldn't get hot enough to warp.
good luck and keep us posted
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