Hi there,
Thanks for taking the time to provide this suggestion.
The EU provides a publicly accessible and free service with a SOAP API which WHMCS can perform queries against in real-time: http://ec.europa.eu/taxation_customs/vies/
The nearest equivalent I was able to to find in the US was a Taxpayer Identification Number Service which (after completing a registration) allows TINs to be validated via a web-form on the IRS website, or uploaded via a text file: https://www.irs.gov/government-entities/federal-state-local-governments/taxpayer-identification-matching-tin-tools
Is there an an API that accepts automated submissions, ideally without requiring registration, that could be used?