Yes, you can cross the border to mail things. People do that on the U.S./Canada border. BUT - - -

1. If the package is undeliverable, they won't return a "domestic" package to your American home address (so this works best for people that have offices/residences on both sides of the border)

2. In crossing the border, you are supposed to disclose that you have merchandise that will not be returning home with you, which will generally prompt a visit to the customs office. In Canada, they charge duty for bringing merchandise in, they may question whether you have necessary licenses and permits to import into Canada, or they may impose a bond to assure you take back out anything you brought in. As a result, people should avoid bringing large amounts in and they would have to violate the law and fail to disclose. I never recommend breaking the law.