You committed to buy an item by clicking the 'confirm order' button. They're T&C's clearly state that once you've clicked that you want to buy they will not cancel orders unless they are out of stock. The only option for you, and not just 7DayShop, is to return the items when they arrive, at your own cost, as you incorrectly placed an order.
To be honest it is not their problem. They run a very small team and keep prices very low comparably to the competition by doing this. They often take 3-4 days to reply to queries and don't often read comms, sure, but whenever I have had a problem and they are at fault they have been very helpful to sort it out.
DB