Pep Boys 80,000 Miles tires last 20,000 Melbourne FL

I bought all 4 tires last July in 2014. I put approximately 20,000 miles on normal roads and highways. My van was maintained by another mechanic and tires rotated and checked alighnment all the good stuff as this is my work vehicle.  I recently had my mechanic tell me I'm almost down the wear mark across the board on all of them and would need them replaced within the next two months. So I brought the Van back for the warranty ... I was told my mechanic wasn't taking care of my vehicle and they wore wrong and my van was tearing up my tires.  And I said no my mechanic is down the street and knows you.. and the men all looked at eachother and I left. I called my mechanic and told him and quite upset he contacted pep boys in melbourne. I don't know what he told them but then they offered me 67 dollars a tire of warranty and it would cost me 410 to replace them.... it had only cost 500 to get them in the first place.   I told them that was unacceptable .. so I called corporate.  No response for a week, then the manager calls tell me 62 a tire and it would be 450.00  so I call corporate.. no response. So then I get a call from manager maybe I can just take the tires with me and get my own install... I said we shall see what that costs.. no response another week goes by I call corporate.... 4 days have gone by.  I now have less than a month left to figure this out before I need to get new tires.  Keep in mind also that a few months into these tires I was getting a vibration...... my mechanic checks ... all 4 tires out of balance.. I call pep boys at the store and they said they had a calibration problem and didn't know how far back it went... but never told anyone of course.   This is a complete rip off .. as a single mom who needs this van up and running. I expected the tires to outlast the vehicle . 20K is not 80K .. and 1/4 of 500 is not 450!!!