I'm not one to buy souvenirs, but these are things I like to pick up when in Thailand :
Crystal Deodorant
Although you can buy natural mineral salt deodorant outside of Thailand, it's quite cheap here (60+ baht) and you can buy ones with aloe vera, mangosteen, or other added ingredients - usually available in drugstores or supermarkets. The last one I bought lasted a year, and I found that it works quite well to prevent underarm odor, even in the tropics.

Simply get the stick wet, use it on your armpit like your normal roll-on antiperspirant, and give it a rinse under water.

Cut-out Statue Postcard
Instead of just sending a postcard, you can get these Ramakian Postcards for 129 baht at 7-11 or other shops listed on their official site. There are a few types of traditional Thai characters or decorated elephants that can be cut out and made into mini 3D statues. I think because each one weighs 30 g, it costs 30 baht to mail abroad, but check with the post office.

There are so many delicious packaged snacks with flavors that are only available in Thailand, ranging from just 17-30 baht. Ones in the photo are some of my favorites :

Popcorn New York : sour cream & onion / cheese

Lay's Miang kham-flavored chips

♥ Grilled salmon-flavored fish snacks!

♥ Thai rice cakes : cheap & good!