If you sell products, you probably know the tax rate for your state. But what about for all the other states you might be selling in? The Tax Data Systems API has that figured out. Using their API, you can insert their look up system into your software to incorporate the answers into the calculations for your customer's invoice.
In the most expensive configuration, Tax Data can validate your customer's address.
In business since 2003, Tax Data Systems boasts an impressive list of clients from Disney to Walgreens. It also covers Canadian taxes, a somewhat more complicated tax environment with three components to track: GST, PST, and HST. Taxes are indicated when due on shipping and handling.
Developers have the choice to combine that shipping and handling tax, or have it taxed alone. Tax Data Systems measures right down to county level taxation, where it exists.
For a user activating a request, they only have to enter the zip code. Tax Data handles the rest. You could, of course, maintain the database of taxes yourself. But you'd have to keep on top of a never ending series of changes.