The key independent variable which determines tax rates is the geographical location of the customer. In this scenario, someone has to conduct the appropriate research to then input the applicable tax rate in the customer table.

Theoretically, one should create a master table based on zip code that has the sales tax rates. Then, create a one to one or one to many relationship between the zipcode in the Sales tax table and the zipcode in the customer table. This way, the key independent variable (location) is driving the sales tax rate, instead of a person figuring it out and inputting the sales tax rate manually.

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Reply from Richard Rost:

Here in the US, your sales tax is determined by the state you're in, but only if you have a physical business presence there. For example, I'm in New York State, so I only have to collect sales tax to customers from NY. If someone from California buys my products, I don't have to charge them sales tax. They're responsible for paying it themselves to their own state (called a Use Tax).

Furthermore, my county (Erie) has a set sales tax rate, which is 8.75%, so I charge this to everyone from NY no matter where they are in NY. The "point of sale" is my address. Even though different counties have different tax rates, that's not my problem UNLESS I have a physical business presence in those counties.

So... if you have multiple stores, then YES, you need to track the sales tax rates for those stores. Otherwise, you only need to worry about ONE tax rate, and whether or not the customer is in your state.