Online contract billing issues

I receive no clear answer from customer care agents wrt the billing for the online contract I signed up for. 600gig month to month @ R399 per month. My first invoice indicates R926.71. A prorata amount was demanded and paid upfront, however the device/service was received 5 days later. They've denied a refund or credit for that portion of the upfront payment that was demanded. But they will not explain the R926.71 billing. Anybody else out there with the same billing issues? It's horrifying when customer care agents provide vague answers, to only the questions they feel like answering...