When I turned it over to open it, this line was mysteriously written on the back.
And inside was, as advertised, the exchange envelope.
Turns out the post office had delivered it straight back to Alyce - I guess the sorting machine must have read the return address on the back, even though the zip is larger and clearer on the front.
As a thanks for the bonus envelope, I sent Alyce #279, another one from an old Rothko calendar.
I even brought myself to part with the second of my sheet of Earthscape stamps.