The treasure has a quiver yesSince NPCs don't use up ammo, they shouldn't need an item that allows them to carry more ammo.
If they did, the player could get such items too easily. If they do carry one, it should be RARE!
But my preferred solution would be to not have them carry one at all and allow them to be found only from special places.
This could be your Bridgetown treasure chest or (especially!) the Oranjestad Indian Trader (who currently isn't an Indian trader!).
