"sent in writing" is correct and usable in written English.
You can use it when referring to written communication, such as a letter, email, or document sent through the mail. For example, "I sent my application in writing last week."
Any "outstanding matters" had to be sent in writing to an address on Fulham High Street, it said.
The company declined to make Mr. Wang or Mr. Kumar available for comment and asked that questions about its accounting practices be sent in writing.
In a previous statement, Cambridge said at least one response from that professor had been sent in writing to Italian authorities.
"I've already sent in writing a request to the appropriate official of the Senate to send it over, so I assume it will be done Tuesday or Wednesday," said Mr. Lott, a Mississippi Republican.
Petitions for pardons are usually sent in writing to the White House counsel's office or a specially designated attorney at the Department of Justice.
Ms. Brooks was asked to appear before the committee at its 2009 hearing but refused to do so in person and instead sent in written testimony.
They also permit senators to be counted as voting at committee meetings they miss if they send in written votes.
He denied the charge and an Independent Regulatory Commission made the decision after a paper hearing, which meant the Scot chose to send in written evidence.
If they agree to participate in the study, they are asked to send in written informed consent via mail.
That said, I kept thinking: what if this letter was sent in by a woman, writing about a man?
(Imagine all the time left over for post-speech commentary had there only been cable television!) But Thomas Jefferson thought the practice reeked of British monarchy and sent his in writing.
