If there's one thing you get a lot of shopping in Japan, it's those plastic shopping bags with handles. You get them in all shapes, colors and sizes. To reduce waste, I highly recommend that you try to carry a cloth tote or a backpack and refuse the plastic bags.
Unfortunately, shop workers and clerks in Japan often deal with purchases in a trance-like state and will often hypnotically either pack your items in a bag or cram one into you shopping basket for self-bagging even when you tell them you don't want a bag. If you give back an unwanted bag, 50% of the time, they'll throw it out anyway because they've wrinkled it up and can't offer it to another customer so prompt refusal while making eye contact (to be sure they're paying attention) is important.
Sometimes, you actually do want these bags because they are good trash can liners for smaller bins or are useful in bagging up raw garbage. The larger of the commonly-distributed shopping bags fit 6 of my 7 separated bins well so I do keep some on hand. In summer, it is especially important to toss the liners on your trash cans frequently because they will attract roaches if there is any small amount of food or moisture in them.
At one point in time, my husband and I had bunched up and crammed so many of these things under our sink for later use that it took up about 1/4 of the available storage space. Since then, I've gotten much more efficient at firm refusal but also have a better way to store the ones I do have. Here is the tidy way to store plastic shopping bags:
Smooth the bag out and fold it in half.
Fold it over again then turn the bottom corner up to make a triangular shape at the bottom.
Fold the triangular fold up to make another triangular fold and keep folding in this fashion until you reach the handles at the top.
Tuck the handles down into the little pocket and it makes a tidy storage packet.
You can store these more easily in a drawer or a basket and it's easier to pull one out when they're in this format.