Daily interest charged for late payments

I have a current account with the Foschini group and this is my dishearting feeling I have with them. I have had this account for the last +10yrs and have never had this type of encounter. I have an instal due of +/-R350 but amount due was R220 which means I am currently in advance for the last 5 mnths. Yes but with Foschini because i only pd on the 7th the consultant at h/office advises that I made a late payment and they would charge me daily interest and interest charged was R40 as I was 7 days late. I did not even know that they changed there due dates for payments from the 7th to the 1st. Not even a letter or sms or call to say im in arrears. They are quick to send me an sms advising that I have R***xx available to spend or an sms advising due to an excellent account they have increased my limit. So how many other people is this happening to...