Is there any custom Buy M + N for Y amount promotion out there, where M and N are different and cannot be the same?
What I am after for example is a variant of fixed price but you have to different entries for the promotion to apply. The one that comes with episerver allows you to chose the same item.
I don't know if other partners have done this, but the point of the new promotion system is that if something is not available out of the box, you can implement it with ease. I think you can base on an existing promotion, but override GetFulfillmentStatus and make sure both M and N are in the lineitems
Are you sure that it is GetFulfillmentStatus? That belongs to the condition. And GetFulfillmentStatus it is the FulfillmentEvaluator -> GetStatusForBuyQuantityPromotion() that evaluates the lineitems.
I would say you have to do alot more...
Create a processor that inherits from the original fixed price, and also reflect all the logic and change the GetRedemptions to do what you want (have not gotten there yet). Then also create your own fulfillmentevaluator.
I'd say the safe way is just to create a complete custom discount for this. It's not much of a discomfort, and you can controll how it looks in the admin UI, so it really reflects what the discount does. You don't want hidden rules in the discount that isn't reflected in the UI :)