"To send a letter" means to write a message on a piece of paper, put it in an envelope, write the recipient's address on it, and send it through the mail to the person you want to communicate with. Full definition
Make a point of sending a letter of gratitude to not only your direct contact but also the CEO of the organization thanking them for their business. (inc.com)
You can support this bill by sending a letter of support to the author's office. (2020mom.org)
Be sure to send these letters in as they are still important! (msicredit.com)