We have some custom rules for shipping we'd like to be able to enforce.

Basically, we have certain products that when ordered alone will use calculated shipping rates.

However, if ordered with other products that qualify for free shipping and less than a certain weight, we'd like to include the item with the free shipment.

For example:
If you order a usb cable by itself it may charge $3 shipping.
But if you order an entire computer with free shipping, include the usb cable in the same box.

In the old AC Gold site we added some custom logic that basically moved the cable to the shipment the computer was in at checkout.


I was wondering if there is a way to do this in v9?
If not, where could I add our custom code to handle this scenario?

Our code with the basic rules can be ported over from Gold, but I'm trying to determine the best place to put it so that it will be included in:
Checkout, Admin Create Order, and Shipping Estimate.

I appreciate any guidance in advance.