Extra debit order without valid/understandable reason

I would like to share my frustration, not really as a warning to not use Vodacom as your service provider because in all other aspects I have always been happy with their service, but as a warning to double-check your debit order details and whether you are paying what you should.

I changed my debit order date from the 1st of every month to the 7th and when my debit order went off on the 7th it was double the amount from what my contract with Vodacom states! I immediately contacted customer care, they’ve tried to explain it to me, but I just do not understand how simply moving your debit order 6 days (the previous month's invoice was paid already) warrants a second invoice for that month (this is the reason they gave me). So in short, my Vodacom contract is 12 months - I will now pay for 13 months, so where will I get the benefit from this "extra month" - how is there suddenly an extra invoice. My telephone contract is simple, I pay a set amount once every month, it makes no sense to me how moving a debit order date doubles the amount, as the invoice for that month was still paid, just on another date (this date and other changes were done and approved by the Vodacom team, they mentioned nothing of a double invoice if I knew I would have never changed it!)