Nice basic one this. Where Carter makes a phone call to buy the heroin and post the footage of the film to Scotland Yard. Now a bookmakers. Interestingly, in the timeline of the film (and as mentioned in the book), this would be set very late on a Saturday, so it would be unusual for a Post Office to be open at that time in 1970. In the novel, Carter gets the postmaster to open especially for him.
As it was in 2008:
Pelaw would be a strange choice to go to to post a letter after exiting the car park – it’s a good distance from Trinity Square and no doubt there are a number of other post offices on the way. It could just be they picked this location for another reason, but an interesting point nonetheless.