In transit does not mean you'll get it today as in transit means that the package is on a truck and going down the highway or in an airplane traveling to the next sorting facility or to your local post office.
Once the package arrives at your local post office and is ready for delivery the USPS tracking will update to say arrived at post office and then out for delivery.
After the USPS tracking says out for delivery then it means that you will get the package that day.
USPS tracking is very accurate in most cases.
However there can be some delays in the USPS tracking updates.
When tracking a USPS package, the tracking only updates once they are scanned so it can take awhile for the USPS tracking to update and show where it's at.
Once the package leaves a post office, the package will get a scan indicating it has left the post office, then once it gets to a sorting facility, the package will then be scanned again and say it's at a sorting facility.
Once the package gets ready to leave the USPS sorting facility again it will say that the package has departed the sorting facility and will update to say that it's in transit to the next facility.
After your package has reached a sorting facility that is near to your address, then it will say arrived at a USPS destination facility.
Then once the package gets to your local post office and is ready to be delivered it will say arrived at post office and then out for delivery.
Finally once the package has been delivered, the USPS tracking will say out for delivery.
It can take 3 to 7 days for some packages to reach your address depending on how far away the package is.