They are not writing the letters, they are looking at Peter’s yellow hat and red socks.
Naturally this requires their full attention:
the hat, a small but rakish panama
being of a brilliant Van Gogh yellow,
the socks, blazoned with a griffin rampant,
an emphatic, even Risorgimento, red.
Peter, known to his friends as Giovanni,
promptly uncrosses his legs,
tips his hat a little over the right eye
and hurries to the aid of the young American tourist
who has just got up from the next table
and who stands, her hair so delicately
disordered for a moment
that his hatless friends, straightening their socks
of unheroic cinnamon,
decide to leave their letters for another day.