SE1 will lose one of its two Crown Post Offices when the Blackfriars Road branch is privatised next March.
Now the company says that the branch will be taken over by ZCO Ltd from March 2018.
The private operator would sell a wider range of cards, gifts and stationery alongside Post Office services.
The Post Office is asking customers and interested parties to give their comments on the proposal in a six-week public consultation that runs until 15 January 2018.
The same services as currently will continue to be available with the exception of the Biometric Enrolment Service for the Home Office.This service is available at London Bridge Post Office.
Opening hours will be extended to include Saturdays.
"We are making sure we take the right action to sustain services for years to come," said Roger Gale, Post Office Ltd's sales and trade marketing director.
Elsewhere in north Southwark, the former Crown Post Office in Walworth Road transferred to a franchisee this month. The self-service machines have been replaced by a counter selling scented candles.