Why would Customs Posts be on a border?
To collect the duty and other taxes due on goods entering.

Now on behalf of HMFC and UK Gov I collect VAT and make a return and if necessary a payment every quarter. I also collect PAYE and make a payment if necessary every month. I submit a Corporation Tax return annually and any payment due.

All this is done on trust.

Since 1990 I have had 3 HMRC visits to check my accounts. All were pre 2000. Since then I have been asked to send down my books for a stated period in around 2007.

I have been "clean" in those checks and maybe they now see me as low risk and have bigger fish to fry.

But the point is, the technology solution may not even be necessary if our civil servants used their noggin.

Why couldn't I declare an import duty and VAT on imports along with my quarterly VAT return?

That would end the debate.