; and (2) may be provided through regular United States mail in cases in which the e-mail or phone number of holders of United States passports is not available. (c) Opt out The pilot program described in subsection (a) shall allow for passport holders to opt out of receiving notifications under the program. 4. Information relating to use of and application process for passports at the time of booking international flights to or from the United States (a) In general The Secretary of State, in coordination with the Secretary of Transportation, shall coordinate with airlines and airline ticket booking companies serving the United States and offering international flights to or from the United States to provide information relating to use of passports, including the minimum length of passport validity required to enter the destination country, and the application process for passports to customers at the time of booking an international flight to or from the United States. (b) Form of information The information required by subsection (a) should be provided through electronic mail (e-mail), Pop-up ads or Pop-ups, directed links on the airline’s website, or other electronic means as appropriate and include a mechanism for users to acknowledge they have read the information before the transaction can be completed. 5. Plan to improve tracking of proof of citizenship documents (a) In general The Secretary of State shall develop a plan to provide greater transparency and improved tracking of documents to provide evidence of United States citizenship and proof of identity for purposes of the application process for United States passports. (b) Requirements The plan required by subsection (a)— (1) should ensure that documents described in such subsection are returned to applicants using a trackable delivery service, which could include providing applicants the option to pay a fee for shipping via UPS, Priority Mail, or another method that provides for tracking of the documents, and ensuring that applicants are informed of