I'm in Thailand too and I never used those shipping service before (because of those 'keeping fees')
I better take my risk of losing my item than paying for 'keeping fees'
If my memories about our duty is right, I remember that we have to pay about 40% of the cost of the item which costs more than 2,000 THB (~60USD). But I'm sure we could request them to check 'gift' box and write down that it cost like ~30USD. which kinda help much.
Or you could use another shipping service which you could find easily by googling. They often comes with duty clearance service too.
If you really need some help just PM me. I'll suggest some of those who offer this kind of service for you =)