Which of the following is included in Extra services provided by USPS?

Proof of mailing is included in the Extra services provided by USPS because it offers customers verification that a piece of mail was sent and can serve as evidence in case of disputes regarding mailing. This service is particularly valuable for people who need to confirm that a document was sent or to ensure compliance with legal requirements.

In contrast, free shipping is not an Extra service, as it typically refers to promotional offers that may not always be available. Gift wrapping is not a standard service provided by USPS, as they focus primarily on shipping and mailing rather than packaging or gift services. Next-day delivery, while a specific service option that some USPS products provide, is not categorized as an Extra service. Instead, it is part of the shipping options that may include various guarantees for delivery times.