I've heard rumors that free delivery can have an estimated value and be added to the total too
Happened to me once, stupid broker, got shipping price from Fedex and added on top of all. Although shipping cost was already in the invoice.
I (presumably also), would get annoyed, when that happens.
It seems such a rip off that when the VAT/duty bill is, e.g. £4 on a £20 item, the post office charges £8 more, just for charging the £4 in the first place.
With Brexit, it is possible the UK has more trade deals, with more countries. So that some/more countries (outside of the EU), DON'T need to have any duty/VAT paid, so the standard country charges (VAT etc) of that country are enough.
I.e. like buying from the EU now, where there are no extra Duties etc to pay, as standard (VAT rates do vary in different EU countries) EU VAT (of the sending country), has already been paid.
On the other hand, Brexit might end the free within the EU postal rule system. I guess we will have to wait and see what gets agreed, after Brexit.