It is funny to see someone putting US, China, India and Afghanistan into one comparison. This shows how ignorant this person is about the economy.
The major export of US are: high-end service and high-end manufacturing goods. In most of them, US is either the only supplier or leading in technologies. Currency value has relatively less affect on these businesses.
The reason that China keeps her export growing is that China has been moving up the value chain. In other words, even though Chinese goods are becoming expensive, the quality is also getting better. They always keep their price lower than the same class products from their competitors.
Afghan, well, that is a laughable argument, they are busy fighting the civil war now. What do you want them to export?