USPS First-class mail is not eligible for inquiry according to the rules of USPS.

Please note that for some packages sent via USPS First-class, there won't be any tracking information once it goes through U.S. customs. You may need to check your packages in a local post office once it is ready for delivery.

Since it doesn't insured, we cannot ask USPS to track it or file a claim. So for some high-cost items, we don't suggest you to choose first-class.