Use checkout and address validation to check shipping/billing addresses, block PO boxes, require house or apartment numbers, and validate postcode/ZIP, email, and VAT/tax ID. Run phone validation at checkout. Build custom validation rules with keywords, regex validation, field validation, cart amount, quantity, weight, products, customer tags, B2B status, country, and shipping-zone conditions. Shoppers see localized errors so they can fix issues and continue checkout.