Amazon Prime Day officially got underway Tuesday, with the summer sales event slated to last through July 11. Competitors Target and Walmart are also holding multi-day sales events.

To help you shop smarter, the Hearst Television Data Team has developed the Prime Day Deal Checker tool. This tool compares a product's price with its lowest price of the year and the last 30 days, then rates the deal as very good, good or not good.

That means your local sales tax rate — ranging from under 5% in some states to close to 10% or more in parts of Alabama and Louisiana — applies to many Prime Day purchases. For example, if you grab a $500 laptop online and live in a place with a 9.5% sales tax, you could pay an extra $47.50 at checkout.

The Dow, S&P 500 and Nasdaq remain volatile as trade deals and tariffs keep investors in limbo. Amazon’s Prime Day met with fierce competition as Walmart, Costco offer their own deals. Oil inches higher.