How long before USPS considers a package lost?
The Postal Service does not use the word that way, it uses day counts. A Missing Mail search request can be submitted starting 7 days from mailing, and a lost-article claim opens at day 15 on Priority Mail and USPS Ground Advantage, day 7 on Priority Mail Express, per usps.com checked 2026-07-28.
Do I have to run a Missing Mail search before filing a claim?
The two are separate processes and neither is a gate for the other. Running the search from day 7 is worth doing because it costs nothing and can end the matter, but the claim window opens on its own schedule and closes 60 days after the mailing date regardless of what the search is doing.
Can the buyer file the claim instead of the seller?
For a lost article, DMM 609 section 1.3 allows either the mailer or the addressee to file when that party has the mailing receipt. The seller normally has the stronger file, since the label record and the purchase evidence usually sit on the seller's side.
What if the parcel turns up after I filed?
Tell USPS through the same case and stop the claim. Recovered items are rewrapped and sent on to the address given in the search request, per the USPS missing mail pages checked 2026-07-28, so a delivered parcel and a paid claim on the same shipment is a problem worth avoiding.