Really the relevant Supreme Court case was Wayfair which required sales tax collection (overturning Quill) for all businesses over a certain revenue(?) in 2018. [1] Sales tax collection was sort of an Amazon-specific patchwork by state previously. (And, yes, in general buyers were supposed to pay a usage tax on out-of-state orders but most individuals almost certainly did not.)

[1] https://en.wikipedia.org/wiki/South_Dakota_v._Wayfair,_Inc.

That wasn't unique to e-commerce though. Pre-internet, if you ordered something from a catalog and the seller was out-of-state, they did not collect sales tax. It was up to the buyer to report and pay the tax, which of course nobody did.

>>> I think they didn't have to collect sales tax on any cross-state transactions

That is not how it worked (or works), in the US Sales tax is owed by the consumer, just like income tax. Amazon nor any other retailer "owes" the tax, they collect it on your (the customers) behalf and submits it to the state. In fact most states pay the retailer to do this as they get to keep a small part of the collected tax.

Before some TERRIBLE court rulings in 2017, Sales tax was required to collective if your business had a "legal nexus" in the state which generally meant you had a physical location in the state you were required to collect sales tax on the sales you made to consumers of that state, so even "cross state" shipments had to be taxed if you say had a office in that state but your warehouse with the goods was in another state

If the sales tax was not collected by the retailer, you the consumer were suppose to pay the tax yourself, most states have a self reporting question on your states income tax return form.

Many people did not do this, so the states started passing laws attempting to "close this loop hole" aka the Constitutional Power grated to the federal government to regulate interstate commerce

Sadly the Federal Courts are TERRIBLE at applying the commerce clause of the US Constitution in its plainly written meaning, as they allow the federal government to use it regulate completely intrastate commerce (which should be unconstitutional) and now (as of 2017) allow the states to regulate interstate sales which also should be unconstitutional by allowing states for retailers that have no Nexus in their state to collect sales tax on sales to consumers in the state

This is also why Amazon, Walmart and others are pushing for Congress to come up with a Interstate Sales tax regulation to override the patch work for some 3000 taxing jurisdictions that make up the US sales tax regulations