Hi there,
As a retired Customs and Excise Officer I can tell you reliably that if the UK confirms a trade deal with the EU then transactions between the UK and Europe will remain free of import duty and additional UK VAT just as they are now. If the UK does not confirm a trade deal with the EU then transactions will be subject to WTO terms. This will mean the imposition of the relevant scale of duty plus UK VAT of 20%. Similarly goods going out of the UK into the EU will be subject to the same duties and VAT relevant to the country of destination. Other regulations, conditions and restrictions may apply.
Remember that when importing from a country currently outside the EU, such as the USA, then duty and VAT will be applied to the amount shown on the commercial invoice supplied with the goods including the charge for shipping. Importations are also often subject to merchant's charges if a merchant processes the importation for you. Any trade agreement with a country usually involves the determination of the rate of duty (if any), tariffs, import restrictions and conditions and quotas.
Finally, when goods are imported into the EU, from wherever outside the EU, then duty and VAT is applied. Once applied in the country of importation then they are available for free circulation without further charge.
Having said all that, not all importations are charged with duty and VAT. Surprisingly, during this Covid-19 pandemic I have heard that many importations have been cleared without paying duty and VAT. I have just experienced that myself when importing a set of the new Kinman pickups; despite the commercial invoice and value of the goods being clearly visible, the item was cleared without the required payment of duty and VAT. By the way, this had nothing to do with my previous occupation - just a matter of luck !!
Best
Ian