Food from convenience stores can't be good, right ?

That is not the case in Japan. I wouldn't say it superb but it's way better than just "edible". Many of my guests are surprised with the quality, especially of the bento, how beautifully it's packed and how good it tastes. Bento from shops including convenience stores in general are sometimes full of deep fried food to make it filling but some convenience stores have healthy bento, using healthier ingredients and cooked in healthier way. So trust me and try one !
Oh, by the way, they will heat it up for you at the cashier so it tastes even better !