[SamReidHughes]

Sales tax laws are different in all the different states. It's not like there's a lookup table matching state to tax rate. You've got different tax rates on different categories of goods, and different tax rates by county.

[r00fus]

Amazon does the web front for Target, and this is a solved problem (it is a PITA, but not intractable).

[protomyth]

It is only a solved problem for all the jurisdictions that have a Target. Sales taxes actually differ beyond the 50 states. For example, MN has a couple difference that cannot be found with 5 digit zip code. You need the whole address / zip+ and map data.