When you're asked "Shall I warm it up?"
Buying a bento, onigiri, or prepared food often comes with the question "Shall I warm it up?" at the register. A simple yes or no, or a nod, is usually understood; if you'd rather it stayed cold, saying "as it is" (sono mama de) gets the point across.
Chopsticks, spoons, and plastic bags
You're often asked whether you need chopsticks, a spoon or fork, and a plastic bag (usually for a small fee). If you don't need any of these, "I'm fine, thanks" (daijoubu desu) is enough. For how to open an onigiri's wrapper or order a hot snack at the register, see How to use a convenience store, and for grocery shopping, see How to shop at a supermarket.
Saying whether you'll eat in or take out
If a store has an eat-in corner, staff may ask whether you'll eat in or take your food away. Saying up front that you plan to eat there makes checkout go more smoothly.
Trash and eating while walking
Some stores have a trash bin for what you bought there, but public trash bins are fairly rare on the street, so many people carry their trash away to dispose of later. Eating while walking (tabearuki) is something some shops and markets ask you to do only right in front of the stall or in a designated area.
Different tax rates for takeout and eating in
Under Japan's reduced consumption tax rate system, food taken away is generally taxed at 8%, while food eaten on the premises is taxed at 10%. For the same item, you may be asked which applies, and the tax-included price can change accordingly.
Related foods
If onigiri fillings interest you, see "Gohan no otomo," and for the hot snacks by the register, see "Fried dishes."