Bruce,
Kudos for looking at some form of address verification for your website. USPS Web Tools is an excellent choice if you are just starting out. There are a few things to consider as you grow. First and foremost, if you have questions or issues related to the USPS service, who are you going to contact for support? The USPS provides very limited, if any, support.
Furthermore, there is no SLA or other kind of uptime guarantee. This may be okay starting out, but do you really want to couple your systems availability to the availability of another system that doesn't guarantee some form of uptime? If that's the case, you'll be looking at re-implementing with another service provider at some point in the near future.
There are a handful of providers out there that have a web-based address verification API. The pricing varies widely between them. Do a search for address verification API to find the various providers.
In the interest of full disclosure, I am the founder of Qualified Address and my company has a product called
LiveAddress that provides all of these features. You're more than welcome to
contact me if you have any questions regarding address verification in general.
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