Best way to add a notice for international customers at checkout?

Hello! I have a small problem that I can’t seem to find a simple fix for on my company’s Shopify site.

My company based in the United States. Currently, we are only able to ship products to the customers who are also based in the US.

Customers only find this out by trying to enter their country/shipping info and see that US is the only option.

So instead of this, my team would like to add a popup/notice at check out that notifies customers upfront about my inability to ship internationally.

If it’s possible, I would also like to be able to offer international customers redirect them to one of our international partners (we have a site that I could send them to).

Does anyone know if this is possible? I’m sure it is, I just can’t seem to find the exact way to do it!

Thank you :)