Amazon is a bad example, because they already have processes in place to pay the VAT and customs fees and include them in the amount you pay up front - at least when the items come from Amazon warehouses - which they originally used when shipping from places like the US. This isn't exactly a new problem, it's more or less just extending the same rules that apply to sales to and from the rest of the world to a few more countries. (Not that you'd likely know any of this from the coverage in the UK press - they've all got a bit of an axe to grind over Brexit and it shows.)