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Sales Tax Request: Validating VAT ID during Checkout

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Currently, it is not possible to request a customer's VAT ID during checkout, and use that ID to provide tax exemptions for customers who qualify (e.g. using the European VAT Information Exchange System (VIES).
It is, however, possible to add a custom field to the Checkout Page, which can be used to collect the customer's tax ID. Note that the number will not be validated.