You shouldn't have to pay anything extra - the £15 import limit is for the cost of the goods not including the postage (unless it's a big parcel sent by courier like EMS or something, then they do include the postage).
That said, Customs often $%&# up and if they do charge VAT they (or the Post Office) will also charge an £8 admin fee. You can reclaim the VAT (if you can be bothered) but not the admin fee....