Thieves broke into a van from the Tong Road Delivery Office in Leeds on September 30.
Royal Mail today confirmed 11 items of post had been taken during the incident.
A spokeswoman said the theft took place while postmen were out on deliveries.
She said: “We have contacted those customers affected to discuss next steps and express our apologies.
“We are working with the police and would advise anyone with any information to contact them immediately.”