If the item is coming from within the EU then (unless Excise Duty is involved - which basically means booze or tobacco) there will be no customs charges.
If the item is coming from outside of the EU and it doesn't have the proper 'CN22' customs declaration form attached to it, it can be impounded by HMRC. Otherwise . . .
If the item is coming from outside of the EU and is valued at less than £18 then you'll also normally avoid charges (but it could be briefly delayed for inspection by Parcelforce Worldwide staff, who carry out the inspection of packages on behalf of HMRC).
If the item is coming from outside of the EU and is valued at more than £18 you'll probably receive a bill which must be paid before the item can be delivered. (Certain items, such as books, can be imported without attracting charges, irrespective of their value)
It's impossible to tell you how much you might have to pay without knowing both the type of item and its value. Import Duty has to be calculated first. Some items are only taxed at 1 or 2 per cent, or not at all. Others, such as cotton-based goods from the USA, can be taxed at 27%. Then VAT (at 20%) has to be added on. (VAT is charged on the cost of the item, on the carriage charges and on the Import Duty - yes, that really is a tax on a tax!) After that, a further £8 'examination fee' is added on to your bill.
For an example, see my post here:
http://www.theanswerb...2.html#answer-6233769
Chris