By default, in Shopify, there is a boolean flag to mark if the item is taxable or not as shown below screenshot.

This can be updated using the substitution list called SPCTAXCLASSES as shown below. 

All of the above is done by default. Nothing to do from user's perspective. 

For customers who are enrolled in the Shopify Plus plan, they could export the actual Tax Categories from Acumatica to Shopify products as there is a field called Tax Code in Shopify for plus customers. 

Please follow the below entities mapping to do that. 

Result: 

Stock Item in Acumatica. 

Product in Shopify after entities mapping export: