Over £15 and you’re subject to VAT. Duty on anything over £135 but not if the duty comes to less than £9. VAT 20%, duty varies depending on what you are buying. You can look them up on HMRC.
VAT applies to the shipping cost and the duty also.
On top, the courier who delivers it in the UK and takes your money for the fees will charge you a nice chunk for their efforts.
Finally, if delivered in the UK by Royal Mail (not Parcel Force) you might get away with not paying as it a random hit, but will have to be a small parcel item. Most couriers will hit you straight off. Some pay the fees up front and send you a bill.