So here she was: sitting in London, visiting a US web site for a US retailer, purchasing a product for shipment to a US address, and paying with a US credit card linked to a US address.
The result: an unhelpful error message stating "Unsuccessful authorization". Payment declined. Frustration ensued.
Deciding to test my "Internet Borders" hypothesis, my better half telephoned the US retailer help line. The help desk person confirmed that her payment had been denied for one reason only: the web server had (correctly) guessed that she was physically located outside the US when she entered her card details and made the payment request.