computer payments gone horribly wrong

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I make payments to a PC company for my computer. I was told back in October that all I had left on my computer was 900 and something dollars. Well in October I paid 99.75 and then in December I paid 245. From my calculations I should only have 647 dollars left on the computer. Well according to them I am wrong. They informed me today that the amount owing is 1400! What the heck! Even with two missed payments, it should only be 697 and some change (from adding in two service fees, which state in my original contract the only thing they can add is 25 bucks each late pay) I am sick. (not literally) . How the heck can a company get away with trying to tack on over 700 in one month! Is that even legal? I have fired off an email to them, stating that IN OCTOBER they quoted me(and I have the original email to prove this!) that I only still owed 900 and something dollars. and in October they received a payment for 99.75 and in December a payment for 245. WHERE does that leave in an extra 700 some odd dollars? Then I go farting around on the BBB, and sure enough they tried doing this to someone else before. And they also LIED to me. They told me that they would be putting this onto my credit file, to show people I am bulding credit. THEY DID NOT. They cannot legal threaten credit action because they never have put it on my file. But I would seriously like to know, is that legal for them to tack on over 700 dollars to my still owing? I don't think so but i'd love to have your thoughts.