Threatening sms's

I have a Woolworths Visa card which I am busy paying off, this I arranged with the department concerned, R300 monthly by no later than last day of the month. Firstly, I am very livid because they send threatening sms's each and every month, from the 20th onwards. The sms states that if I do not pay immediately that my account will be handed over<br> r to Legal. They can stop, coz I have not missed a payment<br> since our arrangement. Secondly, I am disgusted with the interest they add onto the account every month, that will cause me to pay off on this account forever. I now owe more on the account than was the case 7 months ago, I don't mind the charges but the interest is bull, since I am not even utilising the account. They must stop those sms's unless I am not keeping to the arrangement. I am a student, don't have an income, and for our arrangement, that amount is all I can afford at this stage.