The UN was also giving them subsidized shipping rates via the UPU. And as they exploited the subsidies in increasingly-vast numbers, domestic shipping had rates to be increased yet more to compensate. It was cheaper to ship from China than domestically in the US. Basically a tarriff on domestic goods. What do you expect businesses to do? Even if manfacturing wasn't cheaper, it would still be cheaper overall to manufacture there.

Trump finally withdrew from the UPU in 2018 and ended the subsidies.