Second the Canada Post idea. You can also print out the customs declaration/address label on line and then take it with you to the PO, or just take the 2d barcode on your phone and they can scan it. Forget UPS, use FedEx if you have to for oversize, otherwise Canada Post.
If you are using Canada Post to ship, go online, and print a Canada Post label using PayPal.
You just need to have a payment that has come to your account from PayPal in order to be able to print a label. It gives you a good discount when you use the "Expedited" service. It'll be delivered to an address in the US usually by 7 days.