If you sell loose foodstuffs per gram, drinks per centilitre etc. it could be worth inputting how many grams or centilitres of the product the customer wants into your cash register. This can be "switched on" for your products. You do this in Backoffice.
When you create or edit a product, make sure to check Open Quantity. The cash register will always ask how many grams, centilitres etc. that the customer wants of this product. Don't forget to choose the proper unit before you save the product.