My experience with customs over the years has not been easy they want to make their own rules as they go along how much they think they can get away with, but here in Phuket you can negotiate, ok i wasn't importing guitars get your self armed with some facts and figures,
I had two run in with DHL they were calling me from Bangkok to come and collect the parcel and I live in Phuket I paid for door to door service so you deliver it to Phuket, OK send the parcel back about 2 weeks later I got a call from the Phuket customs they are not shy when it comes to money, OK sunshine Im not playing your stupid games get out your harmonization book and I will give the number to look up, I had already done it so I knew what it was it was only 5% bye now he is well pissed off, maybe you could look to see if there is an harmonization code for guitars, might be worth a look