Harrasing and Incorrect Billing

Good day. I ordered goods from Homechoice in December. The invoice I received upon delivery showed I was billed for R1502.59. A day or two later I received a bill via email (same invoice number) but for the amount of R1631.59. This doesn't make sence. I sent a mail to query the invoice but never received feedback. Then I made a payment of R150 (more than my monthly installment) in December and then again on the 26 th of January. On the invoice it shows that the monthly installelment should be paid before the 1st of each month. Everytime I pay long before the 1st and more than the installment amount agreed but I receive calls form Homechoice to ask when will I be making payment and then after a few days receive sms's reminding me that I should make the promised payment (after I already paid) I don't understand why they are harrasing me to make a payment (even though It has already been done). I know I am responsible to make payment. I am not behind on payments and pay more than my installment amount. If I was behind it would make sence but please don't call me for payments! I wish I never orderd anything from Homechoice in the first place. I am going to pay off the account and close it as quickly as possible.