I've had 3 customers reach out to me over the past few days about receiving a notice that states: This vendor does not sell items to customers outside the US.
UPDATE: Shopify responded (Thanks!) and I reached out to PayPal for assistance. Apparently, my PayPal account settings were set to "Block non-US shipping addresses."
Not sure how this happened but if anyone else has this issue, you can fix it yourself by logging into PayPal. Go to "Account Settings." Then "Payment Preferences." Go to "Blocked Payments" by visiting the "Update" link. Then, under "Block non-U.S. shipping addresses:" select "No."