It must be sent once, at the end of the month.
A little song that a friend sent to me once.
She never once sent a messenger or anything after Mother died.
She is sent home at once, without any administrative hearing or review by a judge.
He once sent me a message and that is all.
His company once sent some 100 million emails a day.
"Belisarius himself said as much, in a letter he once sent me."
An inspector was sent once but was here on a day I had hot water.
She telephoned the boys' school and was assured that they would be sent home at once.
Once sent, your friend has 14 days to read it (presumably, the work is inaccessible to you during that period).