Bromley residents can now post letters, buy stamps or send their loved ones packages on any day they choose.
The Post Office last month announced its WHSmith branch in the high street will be open for business on Sundays.
Services are now on offer between 11am and 3pm on the day, while Monday to Saturday trading times will continue as before.
The move comes as the company looks to improve its offerings and allow the public more time to access facilities.
Head of WHSmith branches John Dutton said: "We want to make it as easy as possible for customers to access our services.
"More and more of our branches are opening for longer and we’re delighted that customers can now visit our Bromley branch seven days a week."
- This is the shocking moment a Muslim mum was told to 'go and bomb the world' after argument in high street
- Revealed: These are food hygiene ratings for every restaurant in Orpington High Street
- Students 'trashed' a park in Bexley forcing teachers and the public to clean up after them
- Eltham teen hit 15 people while driving 69mph in a 30mph area
- Bearded dragon rescued from Mottingham house fire