USPS “Available for pickup”: what it means
“Available for pickup” means your package is waiting at a post office or parcel locker, and you need to collect it.
Why it's there instead of at your door
Either a delivery was attempted and nobody could sign or take it, the package didn't fit your mailbox, it was addressed to a PO Box, or the sender or you asked for it to be held.
What to bring
A photo ID and the tracking number or the notice the carrier left. Someone else can usually collect it with the notice signed by you. Check the post office's hours on usps.com before you go.
How long it's kept
Usually 15 days, after which it goes back to the sender. You can also schedule a redelivery on usps.com instead of going in.
Other messages: every common tracking status, A to Z.