The law isn't relevant, they are a private company and just like FB and You Tube they do not like guns. IF you pay by paypal do not sate its a gun or anything to do with one, I used to put sporting goods or toys. However if it is lost or you get done by the seller PAypal will not help as it is against there rules.
On a similar vein always remove valves from gas magazine or you package may be destroyed, with the valves in they are presurized container and as thus banned by all couriers, Royal Mail and Parcel Force. Nine times out of ten you'll get away with it, but that tenth time can be very expensive.