Rimco Did not keep the word of the contract Texas city Texas

I bought some rims and tires from the Rimco in Texas City Texas last year I financed them to build my credit and I did the deal where if I paid them off in 120 days it was the same price as cash they total amount was 5,400 dollars I was paying 600 dollars a month to pay them off within the 120 days then one month I paid my 600 and then 2 weeks later they said I owed another 600 so they charged me 1200 dollars for that months so I just went ahead and paid them off and they never even put it on my credit so I didn't get any credit for it I will never buy anything from them again and I suggest you don't either