I think the combined order (or is it products ) addon could be refined to give an automatic combination with optional discount.
Much like the “Customer also bought this feature” is it not possible to look at text in the product title and list a similer item?
For example book store - The customer places a book in their shopping cart the system looks at the title - finds a match item with the same auther’s name - then says to the customer “Ya we have another item you might be interested in and if you buy this too you qualify for a 10% discount”.
This could in optional for each item - in or out of matching process
The discount could be optional or a free gift even.