1) You really need at least 1 spare magazine. If you lose the one in gun, or present magazine lays down you're SOL. And it doesn't matter how much ammo & fancy night glow accessories you have for your new single shot.
2) Ammo that fits your favorite gun. or reloading supplies that balance your stock for your favorite gun. i.e., if you already have bullets & brass, 1k primers and an eighth of pound of powder,
, then I'd pass on a 'deal' on bullets and save the money until I found enough powder.
3) Beef Jerky. Lots of beef jerky. And cheap canned Beer to wash it down with.