Which form is used to verify the mailer's piece count?

The selected answer, PS Form 8040, is specifically designed for verifying the mailer's piece count. This form is utilized within the USPS to confirm the number of mail pieces in a mailing and ensures that the piece count reported by the mailer matches the actual count. Accurate piece counts are crucial for billing and processing of the mail, making this form an essential tool for mail verification.

Other forms like USPS Form 1234 and Form 1099 do not relate to piece counts in mailing; Form 1234 has different applications concerning postal service operations, while Form 1099 pertains to income reporting for tax purposes. The term "Mail Verification Form" is too vague and does not refer to a specific or recognized form used by the USPS for count verification. Thus, PS Form 8040 stands out as the correct answer for this purpose.