Either there is a sales tax levied just once in the chain (either at the wholesale or at the retail level), or else there is a value added tax levied at every level in the chain (but each player claims back the tax they paid to their supplier).
This. The retail sales tax is just that, only at the retail level. (If you look at the law, technically it's the business that's supposed to pay that, they just pass it along like good capitalists.)
Businesses pass
all of their costs on to their customers. That's how business works. At least they are nice enough to show you how much of your bill is going to the state.
Also, most (all?) states have use taxes. If you buy something online, and the online store doesn't collect taxes for your state, you owe that tax, and are obligated to report and pay it. In most states, the two are combined into a "sales and use tax" statute that makes it clear that it is the purchaser that is obligated to pay the tax, even if the vendor is kind enough to collect, report and pay it for you.