The United States does not have a national sales tax, such as VAT, and you're not able to get a refund from the Federal Government of sales tax you pay on items you purchase when visiting. However, there are two states that do offer sales tax refunds for international visitors, Louisiana and Texas. States that don't have any sales tax are Alaska, Delaware, Montana, New Hampshire, and Oregon.
Sales tax it not included in the price shown on an item, but is calculated and added at the point of sale. The rate varies throughout the country, as both the State and local government can levy sales tax. You would have to ask in the locale you visit, or check in advance using online tools such as this Sales Tax Calculator.
And, as you experienced and as @phoog points out:
It should be noted that many states that have sales tax nonetheless
give tax-free status to certain categories of items. For example, in
New York State, clothing and footware costing less than $110 is exempt
from sales tax.