Does anyone have a well functioning responsive cart that I can see?

I would love to see an example of a cart that works well without customization. I am aving problems with my checkout for two months now and cant seem to get the issue resolved. Im not even sure how its supposed to work anymore.
Particularly I am interested to see if anyone has a cart that offers both domestic and international shipping. I want to know how its supposed to work when someone toggles between entering addresses in two different countries.

Thank you so much if you are willing to share your example.

Best ,
Ann