Price changes when shopping online

It can be very frustrating when you find a product online and then click through to buy it to be showed a different price. This often happens because the search engines index product at a certain price and have not yet picked up that this price has been put on sale or increased on the sellers website.
Another situation which often happens is that at some of the lager retail selling websites such as Amazon, advertise a price but then moments later the price has changed. It is estimated that on average prices change on amazon every ten minutes. If you have not checked out before the price change then you may be looking at paying a higher price (or lower) than you originally saw. Retails also can sell the same product at a different price to different people. You may notice that when you are logged in to your account the price reduces.
All of the above can be done legally but it is worth double checking the total price it is saying you will be charged at the final checkout stage. If you can prove you bought an item at a set price but were charged a different one then you should complain to the company and ask them to refund you the difference.