Import duty is a complex subject filling many reference books - my first solo trial resulted in a 3" thick manual being thrust across the counter by a very stern customs officer with the implied instruction "Work it out for yourself...".
Duty at worst will be about 6.5%, but often less - different goods attract different rates. You will be charged VAT on value of goods + shipping cost + import duty, i.e. you will be charged tax on your tax. Such a fair system.
Little postal packages may get through unscathed, anything bigger will attract customs attention. Our postal system (Parcelfarce [sic]) will now hold your package on which payment is due and write to you asking for payment before they attempt delivery - really inefficient if you have paid for special delivery or a speedy service (two days from USA, four+ days in the UK waiting for their new system to deal with it).
Air freight and big packages will need special handling and the services of a clearing agent (more fees) who will calculate duty, VAT, airline handling, inland delivery etc. on your behalf. Companies like UPS, FedEx, DHL will do this for you (for a fee) and demand payment before releasing your goods. All adds to the cost of importing from anywhere...