Nick here from Shopify.
It's definitely frustrating that PayPal is not available to you in Thailand. That is something which PayPal would need to change and unfortunately out of Shopify's control. In the meantime, there are several payment gateways you can use in Thailand which will enable anyone to buy from your store using a credit card. You can see the list of the gateways here.
I did see PayPal Express checkout was listed there though, so it should be available for you. It might be worth reaching out to PayPal directly to find out more about this. You can see their support page to contact them here.
Hopefully, this helps and points you in the right direction. Let me know if you have any other questions about it.