Secretly pushed the delivery fee up to R900!

I found a phone at a good price via PriceCheck and then I phoned the number just to make sure the operation sounded legit. It was a good conversation. So I proceeded to purchase the R4700 phone via the website. First it showed R350 delivery fee which seemed a bit steep. I proceeded to payment and the checkout screen flashed briefly and showed the delivery fee had increased to R900 and the payment screen now showed I owed R5600. That's dishonest business practice. I phoned to clarify, but no answer.