There are many apps on app exchange for Salesforce, almost all of them are commercial, but there are free code too. As an example look at the source code of the free component (https://github.com/colemab/SFDC_USPS-Component) posted on Github for validating addresses with the USPS API
The United States Postal Service (USPS) does offer this service via their Address Information APIs: USPS: USPS: Ecommerce APIs (https://www.usps.com/business/webtools.htm) For more support, Check out liteblue.usps.gov (https://liteblue.onl/)
USPS: USPS: Ecommerce APIs (https://www.usps.com/business/webtools.htm)
For more support, Check out liteblue.usps.gov (https://www.usps.com/business/webtools.htm)
USPS: USPS: Ecommerce APIs (https://www.usps.com/business/webtools.htm)
For more support, Check out liteblue.usps.gov (https://liteblue.onl/)