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.