Generally speaking, it is legal to ship pistol magazines in the mail if the following conditions are met:
* The magazines are shipped from a licensed gun dealer to another licensed gun dealer.
* The magazines are shipped in a secure container that prevents access to the contents by unauthorized persons.
* The shipment is declared as "ammunition" or "firearm parts."
However, there are some states that have additional restrictions or prohibitions on the shipment of pistol magazines. For example, California prohibits the shipment of any magazines that can hold more than 10 rounds of ammunition.
It is important to check the laws in the specific states involved in the shipment before sending any pistol magazines in the mail.