In Gun Nac you get to visit a shop between levels to spend the money you've collected in your travels to increase your firepower. You even get to visit the shop at the beginning of the game to get a few complimentary bombs to get started.

But, if you hold down Left + Select as you're pressing A or B to dismiss the shopkeeper's text, you'll be presented with the purchasing menu

And, even though your indicator says you have $0, you actually have $10, enough to buy one thing.

Choose wisely!