I think recognizing it's a different problem than billing and have it as a different entity is important. And of course that knowledge needs to be materialized into a technical block.

Now, I'm totally with you that calling that an "engine" is misleading for 99.9% of businesses. In most cases that's a configuration file, or a business class if they're dealing with more user behavior check, calculations or conversions.

Did you previously get a new-customer discount on a previous order? What promotions are in effect? Are you eligible for that promotion based on your location or customer type? Are there bundle discounts that can apply? If you build an order, can we recommend eligible offers based on what is in the order already?

That'll be a fun config file.