I start with E, end with E,
and contain just one letter.
What am I?

An envelope.. The first E and last E are the bookends; the ‘one letter’ is the postcard sealed inside. A pun hidden in plain sight.

Question

Accepted Answer

An envelope. — The first E and last E are the bookends; the ‘one letter’ is the postcard sealed inside. A pun hidden in plain sight.