Post Office: Yes, I used to bike all the way to Leighton, or to Roman Road, but now I discovered the one in Stratford.
18 counters, if I remember well - but some weeks ago it wouldn't show up with their own branch finder
However, it now does:
Stratford 26 - 28 The Broadway, Stratford, London, Greater London E15 4QS
Still a bit to walk, but for me much better than Leighton....