You need to do a little research...
1) USPS is responsible for address validation. It is their system that will add a new address to the database. Right now about 98% of all addresses will validate correctly(unknown). Specially with 5 different types of postal codes that affects how an address is validated. When it validates incorrectly it is normally an error in the USPS system. Nothing can fix that expect them.
2) GIS information normally runs 3-6 months behind USPS.
3) Tax rates are currently gathered monthly and supplied to clients by multiple accounting companies just for this need. Must all tax changes occur 3-4 months before they go into effect. Even if you miss the date, there is normally a grace period in which you implement the change.
YES all these services today you pay. If you use the USPS will be free, but do not over use it.
That is why I would like USPS to be the tax expert, you validate an address for shipping and they supply you the tax rate. Since it is USPS, they would be "always right", and indemnify the user of service. If an address does not validate, that too will be reported *AND* a tax rate will returned to as a default. We used the closest spelling match, and if more than 1 was found, the lowest rate. This logic passed all states, since it was the best that could be used at the time.
If the USPS steps up, the cost should be no more than what it costs today to validate an address. They are already passing back route information and other unneeded information for normal processing, the tax rate(s) that are in effect would be just a little more.