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If you lost your USPS Tracking receipt, it will not be possible to verify the delivery status of your item without the number from the mailing label or receipt. This is because the only record of a confirmation of delivery is that mailing receipt.

USPS Tracking is included for FREE when you ship with Express Mail, Priority Mail, Parcel Select or First Class Package Service using Stamps.com. If purchasing postage at the Post Office, USPS Tracking is included for free when you use Express Mail, Priority Mail or Standard Post (formerly known as Parcel Post).

Once it is scanned though, they say it can take up to 24 hours to be updated on the site. Most of my international packages via priority mail like that aren't track-able online, I have found. You can take that customs number in person to the post office in person.

On the individual package, for any First Class, Priority Mail or Priority Express Mail package, the cost of tracking is very low. It is only $1.05 directly at the post office counter, or you can add it to your online purchase for $0.23. The tracking must be added at the time of shipping.

Occasionally, due to the high volume of USPS shipments, USPS tracking numbers get recycled. However, once the shipping label is scanned, the "new" tracking information for your package will appear.