This depends on the mail carrier the brand uses for shipments. If the brand uses USPS to ship, using a P.O. Box as your shipping address will be fine.
If the brand does not use USPS to ship packages and you enter a P.O. Box as your shipping address, you will see an error message saying that the product can't be delivered to that address. Neither the brand nor Pantastic will be able to make an exception in this case.