I deleted the post which had a link to an in-country agent, sorry. :/
BUT this is the address you use to mail in a 90 day report
Notify 90 Days Chiangmai Immigration
71 Moo 3 Sanambin Road
Tambon Suthep Amphur Muang Chiang Mai
*****
Thanx
@Ga**
for the address, (y)