I usually put a 10 baht stamp on the return envelope when I send my 90 days notification by post to CW but 3 months ago I put a 15 baht stamp because I had only that on hand and the immigration added another 3 baht stamp and sent me back my Receipt of Notification by registered mail.

So this time, I put 18 baht stamps on the return envelope and I effectively received my 90 days Receipt of Notification by registered mail which is better.

Another unwritten rule ?

Might be.

The accept now 5 baht on the envellope and send it back by regular mail.

Mario is correct, I recently had my 90 day report returned and I only paid 5 baht stamp