Illinois Certificate of Mailing is a document issued by the United States Postal Service (USPS) to provide proof that an item has been sent via mail. The Certificate of Mailing is a USPS form that is completed by the sender and then presented to a USPS Postmaster or Clerk for confirmation. The Certificate of Mailing is then returned to the sender with a postmark that serves as evidence of when the item was sent. There are two types of Illinois Certificate of Mailing: Certificate of Mailing (PS Form 3817) and Certificate of Bulk Mailing (PS Form 3602-R). Certificate of Mailing (PS Form 3817) is used to obtain proof of mailing for individual, non-bulk items. Certificate of Bulk Mailing (PS Form 3602-R) is used for bulk mailings.