Credit that never ends

My name is Betty Baloyi ID no. ********** 533081.I have an account with Qeenspark. During December 2017 when I paid my account they told me I owe them a balance R500 -00. To my surprise I was not owing such an amount. I paid R500-00 and during January 2018 I took a hand back of R374-00 plus R25-00 of a lipstick and on the 3rd of February I paid R150 at Qeenspack store and on the 27th of February 2018 I paid another R150-00 at the bank and on the 16th of April I paid R100-00 at the Standard Bank. recently I received a statement indicating that I owe R344-00. I called the shop and submitted my receipt and still I am not receiving any positive feedback. They told me that after settling my account R55 was accrued on my account at the end of December 2017 and that my bank receipt does not appear on my account. finally they told me that R100- 00 from the bank will be processed on the 16th of May 2018. My concerned is how could I paid so much on a credit of R400-00 and end up paying R800-00. how could I pay an instalment of R344-00 a month for a credit of R400-00 and when is my account going to stop accumulating money. During December 2017 I was mislead to pay a balance of R500-00 of the money that was just increased on my account and when I laid a complained I was told that R55-00 was added at the end of December after I paid the balance.