If I want to send clothes, it is more cost-effective to choose a courier company or a post office. If it is a courier company, how to pack it and how to contact the courier?

Express delivery depends on the region, and the pricing method is 1 kg, and the prices vary from place to place. The price of the post office is weight+registration fee. So, if things are heavy and far away, it is cheaper to use the post office, but it will take a long time, maybe two weeks to get there. If it is nearby, and it is within 1 kg, it should be more cost-effective to send it by express.