I send mine by certified mail return receipt so that I have a copy of when I mailed it, who received it and what date. Have had to use that receipt to get paid a few times with the correct royalty.
Here is an example of a letter. And the most recent W-9 0_Pooling order response 2024.docx (13.4 KB)
IRS form W-9 2024.pdf (137.5 KB)