Customer penalized for billing error

I purchased 2 fun filled products for my 18 month old yesterday 25/11/2015 as his xmas present. After checking out on my shopping cart and paying the amount, I get an email from Yolande saying there was a technical error on their site, I need to pay an extra delivery cost - AFTER I ALREADY SPEND OVER R1000. Is this fair as their website states: \All delivery charges are calculated automatically when completing the shopping cart."Just yesterday it said NO EXTRA DELIVERY COST FOR DELIVERY if you buy a certain amount"