Importing music equipment / synth from UK. Customs?

Hi, 

I ordered something for my music (small keyboard / midi controller type thing keyboard and dials knobs etc..! That kind of thing) from amazon UK. Because I couldn't find it here. 

I've ordered a lot of stuff from amazon books vitamins etc.... And had them sent here. Value declared from sender. Usually 10 to 20 gbp. 

This thing is large and will as usual  use track and sign from UK Post office to track item. 

Question is. Will customs check it? Since it will be in a largish box rather than a jiffy bag. It costs 3750baht on amazon. Will I need to pay import duty? How will I know if its held and what's the procedure? 

I would use DHL or EPS as I did recently to send a scarf to my mother. Value 2500 (Laos silk). Amazingly. UK customs held it for over 2 weeks and asked for 20 quid import duty. Plus EPS cost 1000baht. So I do not trust that if I send it that way it will be effective coming into Thailand. And a Compmete waste of time and money. Was supposed to get to UK in 4/5 days for her birthday. 

So. I will use British track and sign which had always been fine. 

For a large item like this, any advice on customs etc.... Duty..? 

Many thanks

P