A study of 87 real invoices found that invoice spreadsheets often assume fields that don't consistently appear. Due dates exist on only 40% of invoices, VAT amounts are more common than rates, and roughly one in eight line items includes tax in the total while others don't, creating reconciliation risks for bulk processing.
VAT API implementation requires more than simple rate lookups; developers must account for product tax classifications, transaction dates, VAT-inclusive versus exclusive pricing, and maintain detailed calculation records for auditability and multi-source commerce scenarios.