If you search the board through advanced search you'll find several threads on the topic. There are many requirements to do so.
You have to cross in the commercial lanes, you have to pay the fee to do so, you CANNOT use the passenger lanes with commercial goods, you have to have the right documentation, you have to have the tariff codes and more...
Some people have trouble and some people sail through. Don't know why. Perhaps it depends on what you're bringing over and it's value. There are restrictions with clothing, textiles and such. Depending on the mood of the border guard they may make you pay duty on the items as well.
I print my postage through Endicia beforehand. I also get boxes from the post office.
