Doesn’t the seller give you a receipt in email or something that itemizes tax and shipping charges?
But, the original discussion involved demonstrating that you did not
make any purchases where sales taxes were not
Receipts would show that sales taxes were collected on an individual purchase, but they don't provide a comprehensive record of all
your online purchases for the year.
and granted it is a big "if,
" but "if"
you were unlucky and got pulled for a random audit by your state and you needed to show that you had not
made any purchases where sales taxes were not
collected, your state can look at your credit card and debit card records which list every purchase you make on the cards for the year and it would be easy to show that every one of the vendors you purchased from collected sales taxes. A few years back this was a bigger issue than today. These days, virtually all U.S. based online sellers collect sales taxes (even eBay collects taxes). The main exception that I am aware of for photo equipment is if people buy from vendors who are based outside the U.S.
Now sure, if you were a crook and you laundered purchases as @Michael Clark
suggested, you might get away with avoiding sales taxes. But, the point was never to show how to be a tax cheat. My point was proving you were honest, not showing how to be a crook.