The issue

We have item level discounts on two categories where one category has a 30% discount and the second one has a 50% discount. Then we defined an order level discount of 40% when a coupon code is applied. The order of discounts is 50% -> 40% -> 30%.

When we tried to add products from both categories and then apply a coupon code, we got a validation error that it is an invalid discount combination.

The research and the solution

Episerver support sent us a Quicksilver example where this setup worked but with a small difference - 40% discount was an item level discount. We tried to re-create the same setup, but it still did not work.

After some research, I found that Episerver support changed Quicksilver code, which applies discounts. They have changed the exclusion level from Order to Unit when calling ApplyDiscounts on the cart.

cart.ApplyDiscounts(_promotionEngine, new PromotionEngineSettings
{
    ExclusionLevel = ExclusionLevel.Unit
});

The default value for ExclusionLevel is Order. So the combination we tried to apply didn't work. Strangely, this is a default value as such discount combinations are common in e-commerce solutions.

So if you need control of combining different discounts on item level, always use ExclusionLevel.Unit.

Thanks to Cuong Phan from Episerver support for help!