MTN is a ****

I am so disappointed and annoyed as I text this. I have been getting calls from MTN about my contract which is due for upgrade in March. I finally came on the 14th of March for an upgrade and was given a new device and I was gonna pay R350. Now to my surprise I have been debited with R1360😳. I went to the store where I signed the contract and they told me I was penalized for an early upgrade. How is that even possible? I need answers!