The US doesn't have a Value Added Tax, which is the one usually refundable upon departing the country... so sales taxes you pay in this country stay in this country and you don't get a refund. Just remember to treat the tax as an implied part of the price. (And be aware that state and local taxes may vary, so the total price may be higher in one place than in another. New York City adds a few percent on top of the state sales tax, for example.) If you aren't sure how much tax would be, don't be afraid to ask.
Tax Refund: The US generally does not refund tax like other countries. For larger sales, you might want to try state tax refunds, check here: https://help.cbp.gov/app/answers/detail/a_id/373/~/how-to-obtain-a-refund-of-sales-tax-paid-while-visiting-the-united-states
US Customs: You never pay US customs when you leave, they don't care about what you take out of the country. You might have to pay customs in your arrival country afterwards, and the rules depend on the country you arrive in. Most countries have a limit on how much you can bring for free, typically in the range of 500 $, but that varies a lot. Also, some countries do not count used articles, so if you wear your new clothing once, it does not count against the limit anymore.