There are different types of proof of mailings available depending on the type of piece being mailed as well as the how granular the proof of mailing needs to be.

Bulk Mail Receipt: The simplest proof of mailing is a postal receipt.  This can be obtained easily for any bulk mail.  The receipt indicates that the post office received the quantity indicated on the receipt and weight verified the mailing.  This does not provide details on each individual piece

Tracking: Tracking can be added to packages which can then be looked up to see their delivery status.  Tracking can also be added to bulk mailings, but this needs to be done prior to the job being processed, includes an additional fee, and is not 100% accurate as it just indicates that last scan at the delivery post office and many post offices do not have the technology to track every piece.  Bulk mail tracking also gives you an idea of where the mail is in the mail stream and it is difficult to track individual pieces.

Certified Mail: Certified mail will allow you to track the delivery to the post office and receipt of the mail piece.  This is expensive and often not cost effective for bulk mailings.  There are options to certify mailings in bulk but it is still very time consuming and expensive.

Use of Permits: Another way to obtain a proof of mailing is to use your own permit which you will be able to see each transaction on the permit through the USPS dashboard.

Seeds: You can also include seed names in your list so that you know the mail was delivered.  This is a good indicator that your mailing is in the mail stream

For bulk mailings, postal receipts and including seed names should clearly demonstrate that mailings went out, but for some mailings it may be beneficial to include tracking and even certified mail to prove that the mail is in the mail stream and being delivered.  For packages tracking is usually sufficient as all carriers have robust package tracking.