Not really. I work out what the price is with all the correct tariffs and VAT added. If it gets through without attracting the fees or less fees than I was expecting I consider it a nice bonus. Some sellers may be naughty and declare lower values for the goods to try and reduce the duty you might be charged.
If you want to look up the import & export duty for anything you can do so here: https://www.trade-tariff.service.gov.uk/sections. Just find the category the product you are look at fits into. once you have the duty add it to the cost of the goods and the postage. Then work out the VAT due (20% in the UK) on the price you just worked out.