Very unhappy Customer!!

Ive upgraded from my Iphone X to an Iphone11 last ,month December, the upgrade was due on the 28 of December. When I did do the upgrade online Ive selected a package where I get the Iphone 11, 1 gig data and 75 minutes over 36 months for R639 per month. ive selected 36 months because of the low instalment.. Now when I went on my app is see that there is a total due of R1899 and the instalment for the package is R1499 !!!! I did not sign up for this because I wont be able to afford this amount... Please rectify my account ASAP!!!!