Generally speaking the onus is on the resident receiving the item. You would be well served to check a site that sells standard capacity mags (the original, intended mags for a gun, not the reduced capacity silliness). If they don't ship to a given locale, maybe you should not either. Of course, if the authorities in another jurisdiction pursue a charge against you for shipping the magazine into their area, the warrant will be (most likely) a misdemeanor and not one that your state will extradite for.
Of course the legal advise you get online is worth every penny you paid for it.
In life, strive to take the high road....It offers a better field of fire.
"Robo is right" Fuzzball