When something is stampless, it means it does not have a stamp on it.
Being stampless is usually not good because it means the item may not have been properly handled or paid for.
Examples:
The letter was stampless, so it couldn't be sent.
He received a stampless envelope in the mail.
The stampless package was returned to the sender.
She was surprised to see the stampless postcard.
The stampless letter never reached its destination.
The stampless parcel was left at the post office.