Overcharge

Dstv is just sloppy and terribly ***********..they have been over charging us for over 5 months now ,we have an agreement of paying 499 on a monthly basis,all of a sudden we pay way more than what we agreed upon and it gets worse every month,so this month we ended up paying 900 instead of 499 and i just checked next month's payment and it says 678 when i call and ask they say i only paid 400 for this month ...im tired of calling as thwy not prepared to assist me ...its tiring really