Interest charges

I received a credit card statement for the month of March/April for an amount of R9 012.30. Payment due date was 6th May 2015. On the 28th April 2015 I paid a round sum of R9 000.00 and received acknowledgement of payment on the 29th April 2015.<br> When I received my April/May statement it displayed an interest charge of R174.55. This is interest on the full amount of R9 012.30!<br> When I queried the charges I was told that due to my fantastic way of dealing with my account over the years they are willing to waive the interest on condition that I sign a debit order on my banking account!<br> I would like to find out if these charges are legal and have taken steps to do so!<br> I am not a happy chap****