There's a marked difference between a genuine single mistake (which happen, as per here) and supermarkets that might purposely hide charges, which, yes, would be illegal. All prices must be displayed, so if one goes into a supermarket face to face, and there is a discrepancy, the customer would let the shop know immediately about it. Thus, there should only be a mistake for a very short period of time. Distance selling online is a different matter. Some people might just cancel the purchase and some may take a time to email the mistake to the retailer. The retailer may also take time to respond electronically. However, since you say you've put them on notice about it, they should amend the price within a reasonable time- and remember that a customer service person can't alter a website in the way a shop assistant can remove a price tag. If you feel they're deliberately undercharging in such a way, contact Trading Standards.
There's no need to shout. The first reply mentions calling, not emailing, which you didn't say you did.