Paid Mails from the Salonika campaign are not common,
Britain and the Empire was Free Postage on Active Service. Mail to the USA had to be paid at current rates, for the USA this was 1 penny
Scan shows a cover from APO SX 4 from the 4th Canadian General Hospital bearing the censor stamp 3 / 4042 allocated to this hospital. Further examined and censored, then passed to Base APO X in Salonika Town where it was dispatched the next day.
Censor 3 / 4048 I believe to be a Base or Chief Censor.
Chris