Do ask the seller, but be careful.
Many sellers cannot ship to PO boxes because they use UPS or another courier which cannot deliver to POs.
UPS and other couriers will charge duty and tax as applicable when bringing your purchase across the border, then add $25 or more as a 'customs brokerage' fee.
USPS and Canada Post is the way to go. If the item is valued under $100 or is used, or is made in a NAFTA signatory, there will be no duty. There will still be sales tax and a service fee of $9.95. But CBSA and CPO often ignore low value items figuring that it would cost more to collect the money than it cost to do so.
