Ash here from the Shopify team.
When creating a shipping zone in Shopify, you're able to add as many countries as you'd like. This mean you're able to add Mexico and USA into the same zone if you choose to, or alternatively you could create a separate shipping zone for each country. When adding countries to your shipping zones, there is a search bar you can use to find the country you want to add. I took a screenshot to show how Mexico and USA can be added separately, or into the same zone:
As shown in the above screenshot, when creating a new shipping zone and adding countries, you're able to add either USA or Mexico, or both. The same is true for any other countries. I hope this helps out, and feel free to reply back if you have any further questions or need any guidance setting up your shipping zones.
Ash, Thanks for the reply. I did that but where do I find what Printful will charge in shipping to Mexico? (example T shirts ) I am using flat rate shipping on all products. The zones I see are: US Zone, Canada, EFTA, Australia & New Zeland, Worldwide, Europe. This is what I see in the dropdown box.