Mattress sinks in center!

I went with the latex mattress after extensive research and wanting to get away from inner spring mattress then trying it out in the store. It was great at first, meaning a few months. After that I started to notice the center of the mattress would not spring back and left a permanent indentation about 3" deep. This has become worse as time has gone by and I'm waking with awful back pain. I contacted the company but they told me I had to contact the store! Then there is a huge charge to pick up/and deliver for repairs. This has a 30 year warranty with the first 10 years non prorated so I suppose the pick up/delivery charge is their way to make more money. This set, queen, cost $4.000.00!! In case you're wondering, No, I'm not an extra large person that caused this mattress to sink. I'm 130 lbs and am the only one sleeping on this set. I'm livid! I'm contacting the store this week and can't wait to see how they handle this.