letter
牘 centers on a written message: a letter, document, or wooden tablet used for writing. The meaning is narrow, referring to a physical piece of writing.
牘 combines 片 (split wood, tablet) with 賣, which likely contributes the sound. The character originally referred to a wooden writing tablet, hence its association with letters and documents.
The left side 片 suggests a flat wooden tablet, and the right side 賣 looks like a complex message being sold or delivered. Imagine a wooden letter being handed over.
For トク, picture a wooden letter that is so special it is kept in a toku (特) box: toku -> トク, and the letter is the treasure inside.