The cooling-off period gives you the right to change your mind within 15 days when you buy from home or through a distance means of communication.
The safest way to cancel such purchases is by filling in a cancellation form and send it by registered mail or hand deliver it.
You should be fully reimbursed within 30 days of of cancellation.
The only charge the trader may impose is the direct cost of returning the goods.
Be careful because certain types of products cannot be returned to the trader.
The best prevention is to take your time before you buy and make sure you really want what you are about to purchase.