I was getting mail sent from my family in the UK to my address in BKK for the last year and a half. Big parcels, small parcels, heavy and light etc. Never had to pay a customs charge. Had a parcel sent in June and I had to pay 500 baht, it was some clothes and shoes. Last week I had a parcel arrive, three T-shirts, and was charged 1000 baht by customs!!Both sent via Royal Mail and charged customs by Thailand Post in BKK