This is the issue that can arise only for international deliveries that have to go through customs procedures on their way to or out of the US. Usually, the standard time clearance takes is 7-14 days depending on the season and which border control office your parcel is assigned to.
usps customs clearance
If you are tracking your parcel with USPS customs clearance delay status will only be uploaded in the database if your delivery is missing a part of the supporting documents, or if the package contents require additional procedures to be cleared from customs. In such case your parcel can be stuck at customs for up to 30 days. That is why if your most recent status update indicates any delays in clearance, be sure to contact customer support at 1-800-275-8777 to find out if you can assist clearance in any way.