BAD BAD CONTRACT PROVIDER

Have been struggling since last year with a refund due to me, then cancelling my contract then i find out i requested a top up in the same month i call them to say the past 3 months i have been billed extra. HOW DO YOU EXPLAIN THAT. According to them i initiated the TOP up of R100. Anyway it is now 3 months later my CREDIT Report shows i am behind on this account so in order for me not to get bad credit ratings i decided to pay this R100 which they say i owe. I will see what happens from there. But according to them i can only ask for a refund on the my complaint within 30 days. LMK