Below is a message from one of our websites that is wanting to sell their products outside of the USA.
How should I respond to them?

I was just informed that when we ship UPS international, even to Canada, we need a commercial invoice and there are customs fees and dues, even when shipping to a residence. How is all that handled in the shopping cart with the homeowner having to deal with those items?

You can get setup with UPS for electronic documents and the invoice will be generated and sent to them, or an additional label is printed that you will stick on the package.
This is handled by the UPS software.

Most sellers leave any taxes/duties to the customer to pay upon delivery. At least that's how we do it.