DStv payments n generating interest

When did Mukti Choice start to generate interest when your paying date it passed? I had to pay R620 this month because they didn't send me the message on what to pay and how much? And now I ended up paying R620. So now when I check my next month's balance they say it's R470 but they say it's R544 what a ******* ...

*** please and their Agents never answer their phones