I have a USD check made out to my name in America. A family member wants to mail it to me, but was unsure of how to fill customs form. Is the value of the check necessary? It is under $10,000. I suggested to just mail it non-registered. But family insist on doing it the 'right way,' raising the red flag for a customs officer to charge me. Advice? Thank you in advance.

 

Just put it in an envelope and send it as if it's a letter.  No need to put a Customs declaration on the envelope.

Do you have a U.S. bank account?   Have them deposit it there.  I assume you can access that account from here.

 

RE Mstevens recommendation  -- yes, that is all that is required but register the letter.   That way you can track it until it is delivered and it will be less likely to get delayed and/or misplaced.

registered letters do not require a declaration, have a couple of folds of paper around the  check so it cant be seen/felt inside the envelope, thats how I have my cards sent from the aussie bank(registered speed mail), you have to sign for it so it is usually very safe and I get it very quick

Yes, use USPS International Tracked with no Customs declaration. Should take 7-10 days if recipient in Bangkok, add-on some days if up-country or down south. The USPS tracking number will appear on the Thailand Post online tracker once the letter has passed Thai Customs and entered the domestic postal system. Handy if it gets delayed or lost. Be prepared to see a few days gap between the last US-side tracking activity and when it appears in the Thai-side tracking activity, sometimes as much as a week in that 'black hole'.

 

Avoid the fastest, most secure method of courier. Even though it won't attract Thai Customs attention if shipped as 'Documents, no commercial value', they usually have their own, legal restrictions on shipping monetary instruments.