I place a two digit decimal in front of each item name. E.g. 0.0 Coffee 0.1 Tea 1.0 Hot Dog 1.2 Hamburger. If I ever have to insert a product between two others I simply use a decimal number that is in between the current numbers of the two items. E.g. If I want to place "Sandwich" between Hot Dog and Hamburger - I enter "1.1 Sandwich".
I had the same issue with my coffee small, medium, large. Large was allway first. I figured if I type in e.g Flat White 5xspace and then "small", next entry Flat White 4xspace "medium", Flat White 3xspace "large", it will come up in order small, medium, large
It's not possible to rearrange the items in the POS screen. However, we are aware that many of our user would like to be able to custom their sales screen, and the Loyverse team is already working in a future update to make it possible.