In most states you can't just add up everything you bought online all year and multiply, because there are different sales taxes for different categories of goo

In most states you can't just add up everything you bought online all year and multiply, because there are different sales taxes for different categories of goods, and some items in an order might be exempt or taxed at special rates, while others aren't. So you need to at least group the items into the different categories. For example, when I lived in Atlanta, there were four categories. Some items were exempt from sales/use tax entirely. Others were exempt from the statewide component of sales/use tax, but still subject to local option tax (groceries were in this category, so they were taxed at 2.2% or something like that). Most of the remaining items were subject to the regular statewide and local-option taxes. Finally, a handful of things (like cigarettes) had an additional excise tax added on top. To make matters worse, you pretty much need a database to determine which items go into which category; the rules around what counts as "prepared food" (fully taxed) vs. "groceries" (partially taxed) aren't entirely intuitive. Another example is that in many states (but not GA), textbooks are sales-tax-exempt, but other books aren't, so if you order from an online bookstore, you need to treat those separately when computing your use tax. The value of the individual items can also make a difference: individual clothing items under $110 are tax-exempt in NY, while those over $110 are taxable.