Libraries for common business logic - best practice?

Hi,

I am working on my first large rails app and had a quick question of
best practice.

It is essentially an ERP type application, tracking customer accounts,
orders, and product inventory.

I am finding I am learning towards creating method libraries
containing certain areas of business logic that refer to multiple
models to achieve their result. They don't necessarily create or
modify existing resources, more make a decision based on some business
rules.

Pricing for example. Given the complexity of product pricing models
with customer specific pricing I was thinking of having some form of
pricing engine that given a customer and a product returned the price
chargeable.

I am also looking at doing similar things with despatch methods and so
forth.

Essentially encapsulating a number of business rules which do not
necessarily belong in the models.

Given the above I had considered creating modules within the lib
directory and requiring them in the controllers as relevant.

Does anyone know of a better ruby or rails specific way of modeling
this functionality?

Thanks, Andrew.