I'm not one to buy souvenirs, but these are things I like to pick up when in Thailand :
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!