Over charging n the name of VAT

I bought 2 protein clear all 1l 217.80 but when I get to the till I'm charged 501.90 they tell me that I'm paying vat of 65.47 of which I dont understand cz the slip shows 217.80×2 which adds to 435.60 which is the 2 items I bought So when I went back to query this price I was told that every customer is required to pay vat in that store of which got me wondering if they go to other retailers to get their stuff and see a price of a product they need is 40.00 and they decide to buy it and when they get to the till the price goes down would they be required to pay 40.00 or promo price bcz the item has gone down.whose responsibility is it to update prices is it us customers or the store its self.NTK I FEEL IS STEALING FROM US CUSTOMERS SAYING ITS VAT WHEN YOU SUPPOSE TO PAY LESS AND THEY CHARGE MORE IN" THE MAME OF ITS VAT" IF THEY ITEM SHOWS 100 ON SHELF WHY SHOULD IT SCAN LESS PRICE ON SLIP THAN WHATS WRITTEN IN THE SHELF WHY DOESN'T IT SHOW 100 ALSO ON THE SLIP, AND THE LADY I SPOKE TOO SHE HAD SOME NERVE TO EVEN TELL ME WITH SMIRK ATTITUDE TO WE HAVE NEVER HAD ANY COMPLAINTS ABOUT THIS BECAUSE THE CUSTOMER KNOW THAT IN NTK THEY PAY BTW WHEN THEY COME.I DONT EVEN KNOW WHAT IS THE BTW FOR AND WHAT IT STANDS FOR.

Their slip:subtotal 436.47 Btw:65.47 Total:501.90