Although cannabis is now legal in some form in more than half of the U.S., shipping it across state lines is still a crime for which people across the country are being arrested. This is true even in cases where it is being shipped between two states where it’s legal in recreational form. That’s because it’s still very much illegal at the federal level.
The U.S. Postal Service is a federal entity, so federal law applies (even if you’re using another carrier, such as FedEx, DHL or UPS). Whether you ship marijuana intrastate or interstate, you risk being charged with marijuana trafficking, for which there are minimum mandatory federal prison sentences depending on how much product has been moved.