The low cost of shipping small packages from China to the United States has made it easy for small Chinese sellers to reach US consumers. Direct sales through eBay and the Amazon marketplace have boomed. Inexpensive postage allows Chinese merchants to offer free shipping to American customers. It isn't fast – international mail from China can take two weeks or more. However, the low price is attractive to both buyers and sellers.
In the past, the cost to ship a 4.4-pound package from China to the US topped out at $5.41 – less than it would cost a US eCommerce seller to ship the same item to a US address.
For example, a Chinese manufacturer could ship a t-shirt that weighs 1 pound (about 453 grams) to Cincinnati. The cost to send this small parcel from China to the US would be less than $2. To ship that same t-shirt from Miami to Cincinnati, you'd have to pay $6.70 for Priority Mail, since most domestic packages over 13 ounces must be shipped by Priority.
Although the US t-shirt in this example would arrive much more quickly, there's almost a $5 difference in shipping costs. This gave the Chinese eCommerce seller a competitive edge on shipping costs. If he can offer his products at lower prices, that's hard for a US-based seller to beat.
Now that the new Universal Postal Union treaty has gone into effect, this equation may change. Small parcel charges from China to the US have risen significantly. It's too early to tell what effect this will have on Chinese eCommerce sales to the US and other countries. However, it puts US eCommerce companies on a more equal footing with their Chinese competitors, at least in terms of shipping costs.
https://redstagfulfillment.com/universal-postal-union-treaty/
Also, the whole CCP is a criminal enterprise that doesn’t respect patents, trademarks, copyrights, or much else, which is a big cost saver.