Hi
Depends how your goods are being sent... WHo were they sent with? If they being delivered by UPS, Fedex or DHL then you are likely to get your goods and then receive a bill a few weeks later. If they were sent by USPS or Parcelforce (USPS often sell the charges to Parcelforce) then you will receive a letter once it is in your local depot - although if the tracking shows it is in your local depot you don't have to wait for the letter and you can go and pick it up (and pay).
All of the couriers charge a fee for doing this - anywhere between £8 and £15 - if it is Parcelforce delivering it to you the fee will be £13.50.
Hope that helps.