While in many states clothing is subject to sales tax, in some states it is considered to be a basic need and thus is either entirely non-taxable or only taxable when priced at above a certain threshold.
These states are:
Detailed information on the tax status of clothing in each state can be found here.
All the other Printify products are taxable goods.
Many states also consider shipping costs to be taxable - further information can be found here.
Please note: Sales tax will be applied on sample orders, since they are not considered to be purchased for resale.