Why do orders imported from Shopee have tax / GST?

The app does not control whether taxes gets added to Shopify orders. This can only be done through Shopify. For most of our customers, we recommend that you turn off taxes completely by following the instructions here

https://help.shopify.com/en/manual/taxes/tax-overrides#set-a-0-tax-rate-for-most-of-your-products

However if you need to enable taxes for your Shopify orders while disabling taxes for your Shopee orders. One thing you could do is to set up a tax exempt customer by following the instructions here

https://help.shopify.com/en/manual/taxes/tax-overrides#set-up-tax-exempt-customers

Then you can give us the ID for that customer, and we can set up the app so all Shopee orders will go to that customer. For more information please read: How do I use the same Shopify customer when importing Shopee or Lazada orders?