Based on the still-unsolved murder of Susan Taraskiewicz, a Boston airline baggage supervisor who was found stabbed to death in the trunk of her car in September of 1992.
Any tampering or theft of U.S. mail falls under the jurisdiction of the United States Postal Inspection Service (USPIS), which is the law enforcement branch of the United States Postal Service (USPS). Their jurisdiction is: "crimes that may adversely affect or fraudulently use the U.S. Mail, the postal system or postal employees." So while it would be proper to inform the FBI of the credit card theft, as credit card fraud falls under their jurisdiction, the tampering and theft of the mail bags would need to be reported to the USPIS.