Apply promotion on any reduced line
I think that the way we apply the promotion is not really what is expected by customer. For now we apply a promotion only if it reduces the unit price of all the valid lines. But sometimes some line may have a more reduced price due to a price list (per quantity) or another promotion that overlap some products.
In case of overlapping promotion, it may depend on the order they are applied which is even more complicated to explain.
So I think we should apply the promotion on any line for which the unit price is reduced.
Those who may want a more restricted application could extend the criteria of promotion selection.