Take the invoice total including postage as the valuation.
(Note that Border Force can disregard declared valuations and declare a value in the UK market)
Expect to pay 20% import VAT, possibly import duty (depending on item classification - could be 3.2%)
Then for the holding of the package pending payment and handling of fees the importing carrier charges their fee which can be anything from £8 to £20
E.g. you pay £100 on the purchase, then get a bill for £23.20 + handling fee
The final cost being between £131.20 and £143.20