PHP / Google Maps API - Easiest method to verify a street address and standardize / prevent duplicates?

I'm glad you asked this question.

UPS and USPS both offer free APIs for address verification.

However, USPS requires that you can only use their APIs if you intend on using their shipping service. UPS doesn't appear to have this restriction. The TOS looks fairly boilerplate and you aren't required to use them for shipping (although I'm not a lawyer so idk).

USPS Developer Kit - https://www.usps.com/business/webtools.htm?

UPS eCommerce APIs - https://www.ups.com/upsdeveloperkit?loc=en_US

List of 7 Address Verification Services - http://www.programmableweb.com/news/7-package-tracking-apis-rock/review/2014/08/26