Technically its a weapon if www.dictionary.com isn't telling lies and you class paintball as combat or fighing or war:
1. any instrument or device for use in attack or defense in combat, fighting, or war, as a sword, rifle, or cannon.
2. anything used against an opponent, adversary, or victim: the deadly weapon of satire.
As for firearms no, because it doesn't use an explosive charge as a propellant.
Thats roughly the price for 1st class recorded, not guaranteed next day, just needs a signature of Recipient to prove it was delivered. If it goes missing, the sender is only covered upto £34.
If its a expensive marker, worth doing special delivery, guaranteed next working day, and covers the sender upto £500 if it goes walkies.