Ownership-Based Discounts

I'd like to be able to offer a discount on one product to customers who already own a different product of mine (e.g. own Mayu → get a discount on Mayu-ni).

Important edge cases this needs to handle:

• The highest discount should only be applied once per customer only if they don’t own the product, even if they own multiple qualifying items — it shouldn't stack or be repeatable. This is to avoid multiple licenses.