Sales tax is a tax added to the price of a retail sale and collected by the seller from the buyer. In the US it is a state and local tax: the IRS refers to “state and local general sales taxes” and describes a general sales tax as “a tax imposed at one rate on retail sales of a broad range of items” (Topic no. 503). Because the rules and rates are set by states and localities, the right rate depends on where the sale is taxed and what is being sold. Unlike VAT, which the EU charges at each stage of the supply chain, a general sales tax applies to retail sales. On an invoice, sales tax is a separate line under the subtotal, showing the rate applied. Example (with a hypothetical combined state and local rate of 8.25%): $1,000 of taxable goods, $82.50 sales tax, total $1,082.50. Common mistakes: charging it on items your state exempts, not charging it where you're required to collect it, and labelling it “VAT”. Check with the state's tax agency before you invoice.
Try it: Invoicing in the US.