Is straightforward: promos is a list of functions. Once you get used to the idea that functions are first class objects, it naturally follows that building data structures holding functions often makes sense.

Best_Promo.py

>>> Order(joe, long_order, best_promo) 
 <Order total: 10.00 due: 9.30>
 >>> Order(joe, banana_cart, best_promo) 
 <Order total: 30.00 due: 28.50>
 >>> Order(ann, cart, best_promo) 
 <Order total: 42.00 due: 39.90>

Promos.py

promos = [fidelity_promo, bulk_item_promo, large_order_promo] 
def best_promo(order): 
 """Select best discount available
 """
 return max(promo(order) for promo in promos)