Address Validation Vendors?

Our ecommerce site is looking for a vendor to provide a real-time, CASS-certified API to validate addresses in a web form in our cart. If anyone has had experiences (good or bad) with particular vendors, please share. I’m leaning toward SmartyStreets at the moment, and looking at QAS (part of Experian) as a second choice, but I’ve looked into a number of others. Thanks for any feedback.

Is it just for one country? If it’s just for one country only, that country’s national postal operator might have an address database that you could license from them, then all address validation could be done locally on the server.

Thanks for your reply. It would be for USPS addresses for sure, and possibly Canada. USPS licenses their database and we’ve considered that route, but to minimize the implementation/maintenance work for our IT, we’re looking ideally for a third-party host that we can just make API calls to. USPS, UPS and FedEx all offer free APIs, but their service level is lower than what we need, so we’re exploring paid vendors.