Sales tax is automatically calculated for each order based on the "ship to" address - more specifically, on the zip code.
The exact sales tax percentage differs, not only for each state, but also within the same state. Sales tax is calculated from the Printify production price (from the final retail price for California).
In some states, the shipping fee is also taxable.
Sales tax will not be charged if the merchant has submitted a valid resale certificate for the given state.