Non-purchased products in shopping lists are sorted by color. The color groups go in the same order you can see the colors in the color panel, when you add or edit a product.
Within each color group products are sorted alphabetically. Like this:
Hint: if you do not want to group your products by color and would like them always to be sorted alphabetically in your lists and products dictionary, you can assign them all to the same color group (or do not assign them to any color group at all).
The product that you've just tapped as purchased, is shifted to the beginning of the bottom part until you close or minimize the app. The rest of the crossed-out products can be sorted either by time or alphabetically. The way of sorting can be chosen in the app settings ("Crossed Items Order" section).