@hellomiakoda You should always see if there are items that weigh the same but cost less, scan those and leave them behind.
example: 100grams of walnuts is 1/3 the price of 100grams of cashews, 1 can of kidney beans is 1/2 the price of adzuki beans
Buy both if you feel, but scan only the cheaper items.

If they won't "train" you or compensate you for your time, you cannot be blamed for "making mistakes" when scanning. You are working for them. It is their fault if "mistakes" are made.