*********ed into paying a wrong amount

We were assigned this company by Vumatel to install our fibre. We were told out first 10m of trenching was free and thereafter R50/m. We were told (after a thumb suck) it was 17m therefore R350. I then received an invoice for R500 excluding VAT which I was never quoted on. After complaining I was told if I don't pay the fibre will be cut off from our wall. I felt *********ed into paying whatever amount they chose I should pay