Tire and Alignment weight problem

I have a 2008 Kia Rondo purchased new. At 23,520 miles it needs 4 tires and a 4 wheel alignment. When I purchased car I went over use, I carry samples weighing 400 to 500lbs,and was assuered this is not a problem. They now tell me its to much weight and car will not stay in alignment with weight. I feel like I was lied to i n the beginning. I rotated the tires less then 1000 miles ago and there was no problem. Neither the agency or Kia is willing to do anything about this issue. This vehicla has a 3rd seat and should hold 6 to 7 adults, more weight then my samples.