Overcharge

I am currently at KFC VANGATE MALL, bought a box clever which is priced R69.90 and includes a small drink but the drink is not available, they said I must change to adding a buddy bottle drink,then they add R10.00 to the cart instead of the difference of the price of the drinks.Thats just dishonest and unprofessional as that is overcharge because the price of the small drink is already included and they should have charged the difference between these drinks. I am very disappointed that such a large company can be so dishonest.