If your package was shipped using regular first-class package service, then you should not contact the post office as long as you are still seeing updates on your tracking status. However, if you have not seen any updates after a week, then go ahead and contact the post office for help. If the package was sent using a guaranteed service like Priority Mail Express, then you should contact the post office as soon as the package is late. You might be entitled to at least a partial refund of your postage.