Can you help me please!!!

I am a mother of a 4yrs old girl. I invest most of her learning stuff with dvd's and books. One time we were shopping at the mall when this Grolier group approach me and my baby and gave a sample for their product in short I avail the products and gave them a down payment all is well co'z they have this receipt and copies. I avail their short term since I was into their promo prize. They have this collector who will go to houses to collect for the monthly payment. The receipt was with a certain amount already and it was typewritten. So I paid for I think 3 months since it was installment. after a year they contacted me and says I still have a balance which is so surprising coz I knew I paid already. Accidentally when we were planning to move a house some documents we had was lost and my receipt was included in that documents.Again after a year I am were receiving letters from their lawyers saying I have to settle my balances or else they will sue me. I don't have proof that I paid already since it was a year ago. I did not move same address although I change my number.There was this inconsistency w/ the statement of account that they gave me and it shows different amount which is why I wonder how come if this was installment it should have fix amount on the receipt right?. PLEASE HELP ME,THE LETTER I HAVE WAS AN ADDRESS ONLY NO CONTACT NUMBER AND I TRIED TO CALL THIS LAWYER BUT HIS NUMBER WAS IN ACTIVE. I TRIED TO SEARCH THE GROLIER'S NUMBER AS WELL BUT NO ONE ANSWERED. CAN I MAKE A DEMAND ON THIS COMPANY TO HAVE AT LEAST SEE ALL THE RECEIVING COPY THEY HAVE TO SHOWS THAT I ALREADY PAID THEIR PRODUCT? WHAT WILL BE A CASE FOR ME IF EVER THEY SUE ME AND CAN I COUNTER CHARGE THEM? IT REALLY BOTHERS ME. PLEASE!!!