Vodacom upgrade charges

I upgraded my vodacom contract on the 1st of July and did not receive any notice on store level at the Cape Gate Vodacom shop of early upgrade cost implications and also signed a contract with zero indication of additional devise fees but then received a bill back dated one month and for two months thereafter with an additional devise fee of R744. 66. After reporting this on store level they logged it with accounts only to tell me 3 months later that I had an early upgrade and that I am responsible to pay the additional devise fee. If this was disclosed when I enquired on my my upgrade I would not have proceeded with early upgrade. I have a written contract with a fixed amount but was charged more than what I signed for.