Composite

Treat parts and wholes uniformly

Composite lets you build a tree where a single item and a group of items share the same interface, so client code can call total_price() on either without checking which it is. A walk package can hold plain items and sub-packages; a group's price is just the sum of its children.

The trick: both the leaf and the group answer the same method. Recursion falls out naturally — a group asks each child for its price.