Use taxes are the most common method of collecting sales tax on online purchases I think. They're disliked because they require self-assessment, but it's easier than asking random Amazon sellers in Washington to collect taxes for Vermont.
Generally, sales tax is (generally) required to be collected if you have a "physical presence" (or "nexus") in the state (this has a specific definition, but includes warehouses, stores, offices, and sales representatives) and is the gold standard; or if you have so much money (the amount varies) in sales per year to within said state. An example, this is a big picture of places where Amazon collects sales tax:
It's something a lawyer can answer better but basically for a substantial portion of Americans they will pay $19.99. Looking at the map, it's probably closer to ~20-30% than 50% I said earlier. My personal experience has been that online purchases never charge sales tax, only shipping tax, but this has unfortunately changed as Amazon now has sufficient physical presence to trigger the nexus law in my state.