[jhh]

Personally I would make models a package rather than just a module (since you sy you have a lot of order related logic). I would create one or more modules in this package, presumably one of those for orders.

Then I would actually create the base logic which clearly belongs to one Order instance as methods on the Order class.

If you have something like "can x and y and (...)" be ordered together I would make an unbound function in your orders module which takes an iterable (Lists etc.) of Orders and works on it.

Note that I am by no means a Django guru or something like that.