Finance charges

I was charged the total finance charges on a 6 month same as cash credit loan when purchasing a computer in store. I paid all 6 months before due date yet they will sent me a bill for the finance charge because it was only 2 weeks past the "contract" date. I was told in the store it was 6 months from the 1st payment due but now they are saying it was 6 months from date of purchase. They still send me a bill every month for 9 months now adding late charges of $29 every month.