Final answer:
The proof of proper postal acceptance for packages bearing only stamps is demonstrated through a postmarked date on the package.
Step-by-step explanation:
The proof of proper postal acceptance for packages bearing only stamps can be demonstrated through a postmarked date on the package. The postmark serves as evidence that the package was accepted by the postal service on a specific date. This is important for tracking the timeline of the package delivery and providing legal documentation of its acceptance.