Sales tax is unique to the United States in the sense each state has an individual sales tax rate that businesses account for on a state level instead of a federal level. Most other countries use a federal taxing scheme on transactions, called VAT, but the U.S. Has the sales tax system instead. Another unique thing about the U.S. Taxing system for businesses and freelancers is that invoices are really required within a B2B (business to business) framework while receipts are often issued when dealing with B2C (business to consumer) transactions.
Our invoices here on InvoiceOcean US are tailored for the U.S. Financial market and sales tax is an option any user issuing invoices form within our system can input. Because of the number of rates due to U.S. Having more than 49 states and some states not even requiring a sales tax, we have opted to allow our users to input their own states' rates into the sales tax field on the invoice.
For a complete listing of the sales tax for your state or the state you are inquiring about for invoice purposes, check out this graph compiled by Tax Foundation using stats from Sales Tax Clearinghouse and State Revenue Department websites and mapping it geographically. Keep in mind that this data is current for the year 2018 and may need to be updated in the future or you may need to find another more current source. Here is also a list of states that have other local taxes you may want to include on your invoices, such as city tax.