An AI agent quotes the price on the page. For a cross-border order that is not the price that lands. This prices the border, cites the instrument behind every line, and refuses to guess the ones it cannot derive.
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Customs duty sits inside the taxable amount for import VAT. If the duty is unknown, any VAT figure is understated by the duty times the rate. An agent can act on "I need this one input". It cannot act on a confident wrong number, and it will not find out it was wrong until the goods are at the border. So the line comes back null with what it needs, the quote reports itself incomplete, and the PayPal order leaves those lines out of the amount charged rather than estimating them into it.