Dissapointed

I received an sms on my phone fro the bank stating that Vodacom has deducted R2677.45 ??? i went to see vodacom in Greenacres ,because how can they deduct R 2677.45 for an top up data contract of R 89 ??? Spoke to one of them,and they told me it is because of windown 10. But still even if it was windows 10,this is a top up meaning that when my data is finished i cant go on the internet. so they told me to phone vodacom customer care who i phone twice and then they told me but this never was a top up,and that is what i owe them ??? but why when i previously finished my 1 gig data i could not use the Internet and now suddenly since vodacom has migrated me from a 1 gig to a 2 gig now it is a open contract??? I never gave any one the authority to change the contract itself and no one explained that my contract will change from a top up to an open contract ?