The most likely reason is because shipments below a certain value were not being inspected and this was a loophole that allowed China to undermine the US by shipping fentanyl ingredients. My guess is that the postal service needs time to resolve this issue, and are choosing to stop processing these packages for now.

The other possible reasons are to impose tariffs on shipments below the low value threshold, to disrupt the Chinese economy which is currently weak, to discourage consumption of Chinese products, or to use as a negotiation tool in the current push to reset foreign policies.