You must have signed up the the Professional selling plan which is £30 a month.
You can downgrade by going to Settings - Account Info - Your services - manage, and downgrade to the Individual plan - where you will pay an extra fee per item sold.
You will not be charged again, however check out the difference between the two selling plans as you may wish to remain on the Pro plan for the other advantages, such as being able to set your own shipping rates, and download order reports etc.