excessive billing

I have a laptop on contract with vodacom for which I pay R305.00 every 15th of the month. In terms of the contract I receive 200mb of data every month.This moth (November) the 15th was on sunday so I got paid on Friday the 13th. Vodacom did not debit the amount on the 15th as per agreement even though I reserved same on my account for them. On thursday the 18th I received an email from them informing me that I owe them R1800.00 and that they were declined when they trierd to debit the amount on my account. What a shock.How did the amount got to that figure when I hardly use the data they alocate me. What a shock! My contract with them is expiring early next year maybe they want to get as much as they can from me before we part ways for good.I' m really disapointed in this company. How I used to love them.But I won't give in that easily I want to get to the bottom of this.How?,how?,how?