About this letter:
( 1839 HMS HASTINGS SPITHEAD 74 GUN SHIP LETTER CARRIED BY 1ST LIEUTENANT RARE ! )

The letter was posted at the receiving house of Princes st.
It was charged for 2d, because it was sent within the town area of London.
It was redirected within the Town Area of London so the total charge was 4d.
The two red postmarks show the timecodes of 2pm and 6pm.
IF I remember correctly the meaning of the times I try to explain what happened, by this table of 1839:

So posted at Princes st before 12 Noon.
It arrived at the principal office and charged for 2d.
It was then sent out for delivery at 2pm.
It was then redirected to a new place so it went back to the principal office.
2d charge canceled and charged for 4d.
And from there it was sent out for delivery at 6pm.