DESPITE protests from shippers, the Intra-Asia Discussion Agreement (IADA) has refused to back down on plans to increase terminal handling charges (THCs) from Sunday. The IADA, a group of 36 container shipping lines which serve Hongkong and other ports in the intra-Asia trade, will raise THCs by US$200 for each 20-foot equivalent unit (TEU) to $1,000. In talks with the Hongkong Shippers' Council (HKSC), IADA agreed to freeze the Hongkong THC level for 17 months until January 1, 1995 and to limit future THC rises to the Consumer Price Index. ''The present decision by the IADA to implement the August increase but at the same time freeze the level for 17 months and cap future increases cannot be regarded as a negotiated settlement,'' said HKSC chairman Chan Wing-kee. This concession, he said, did not alleviate dissatisfaction over the increase. ''We still feel that the IADA should abandon, or at least postpone, the August increase.''