Harassment to pay account

I bought a bed at Beares during January 2015. On/about the 20th I started getting phone calls every 2nd day from the branch reminding me to pay my account. The contract determines that I should pay before the 3rd of every month. Since my salary paid in on the 28th of January 2015, I received daily phone calls asking me when I would pay my account. After I told them that I would pay before the 3rd, I still received phone calls. On the morning of the 3rd the lady phoned me and asked if she can come pick up the money at my work place. Initially I said yes and she confirmed that she will phone before she comes. When she phoned, I told her that I am going into a meeting and that she should not come. Still she did come while I was in a meeting. I proceeded to pay the installment into ABSA as indicated on their account and when they phoned me again I told her that I paid it into ABSA. She said that they don't make use of ABSA. She showed up at my house that evening to collect the proof of payments, which I refused to give to her as I did not have copies for my own records. This is harassment. This month the account shows overdue even though I sent through prove of payment.