How do I validate BTW-nummer for B2B customers?

For intra-EU B2B sales you can apply the 0% reverse-charge BTW rate only if the buyer has a valid BTW-nummer registered in the EU’s VIES database (VAT Information Exchange System). In Magento we add a custom customer attribute btw_nummer (or vat_id if you’re using Magento’s built-in field) to the registration form, validate it real-time against the VIES SOAP API on submission, and gate the B2B-only customer group + 0% tax rate behind a successful validation. We also store the validation timestamp for audit and re-validate every 90 days (since BTW-nummers can be revoked). For non-EU B2B (UK post-Brexit, US, etc.) we use the relevant local validation API or fall back to manual approval.