Complex model scheme design patterns

I am designing a cart system and cannot quite decide how to handle the multiple-codependent model values. This is quite a conceptual question, I hope I can explain it right.

Many properties of a certain Order (and associated Models) record are dynamic (and directly affect each other) and I am trying to make a robust data workflow which would yield the most stable, yet flexible results.

Consider this model schema (unimportant properties are omitted):

- Order [weight, total_charge, tax_charge, shipping_charge, country_id ...]
- OrderItem [order_id, quantity, unit_price, discount, tax_rate, subtotal,  ...]

- Payment [order_id, amount, ...]

If you change any of these properties, all others are affected.
eg. Order.country_id affects Order.shipping_charge and OrderItem.tax_rate

eg. OrderItem.discount affects OrderItem.subtotal, Order.total_charge and Order.tax_charge.

I would also like to be able to make "faux" calculations, without stored data, by supplying only a data array and have all the data rendered.

etc.

I am wondering WHERE and HOW should I properly approach these connections in their respective models? beforeSave callback, afterFind? Both? Make a behavior?

Does anyone have experience with this kind of co-dependence in Cake? How to make it as clean and robust as possible?

Thanks!

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