Which Extra service can provide proof of mailing?

The service that can provide proof of mailing is the Certificate of Mailing. This service specifically offers customers a receipt that serves as evidence that their mail was sent on a specific date. It's particularly useful for situations where a sender needs confirmation of mailing for legal or personal purposes but does not necessarily require the added security features of other services such as Registered Mail.

A Certificate of Mailing does not provide tracking or insurance, but it fulfills the need for proof that an item was actually mailed, which is a primary requirement in many mailing scenarios. Other mailing options like Registered Mail provide higher security and tracking capabilities, but they also come with their own distinct features and costs. Priority Mail and Standard Mail focus more on speed and delivery options rather than providing formal proof of mailing. Thus, for the specific need of having proof that an item was mailed, the Certificate of Mailing is the most appropriate choice.