Re: Er ....
States can set their sales tax rates to whatever they want and some states allow local sales taxes in addition to the state tax. The major complaint is between the states and local sales taxes there are several hundred different rates to charged based on the buyer's location. Note, the buyer's street address might be mislead one into assigning the buyer into the wrong tax jurisdiction as US zip (postal) codes do not always align with the local boundaries.
Compounding this jurisdictional problem is each state has different rules about what is taxable which is another sources of problems. Again, local sales taxes may not follow the state rules for what is taxable. The dissenting Seniles are correct, no matter how wrong Quill was, the correct place for a major policy shift like this is by Congress even if they are less than stellar examples of humanity.
Writing a program to correctly calculate the sales tax is fundamentally not difficult in concept but it is the myriad of local inanities that will make code details a nightmare. This software would have to be updated regularly as new state tax regulations come into effect.