It is in the best interests of both you and your local delivery company to help make sure that you have all the required paperwork filled out correctly before you ask your Canadian courier company to take a shipment to the United States for you. If the paperwork is not filled out correctly, it can end up costing you and your local delivery company money because it will inevitably cause a delay at the border. Cross border shipments are always a sensitive matter and it is crucial that all paperwork is filled out, and correctly, before you should ask your company to make a shipment over the border. If you are inexperienced in this area, talk to your account representative with the courier company about it. They likely ship cross border frequently, and can help to show you the proper forms you need to fill out, and how to go about doing it. Their experience in the area makes them a great source of advice.
The first two things you are going to have to worry about when using a Canadian delivery company to take something to the US for you are your commercial invoice, and possibly a B13A Export declaration form. It's safe to assume that everything you ship from your business is going to need a commercial invoice with it. Even small purchases made over online sites like eBay need this, so always start by including an accurate commercial invoice with your package. Your Toronto courier company, based on what you are shipping, may also tell you to fill out a B13A. If they request that you do this, they should either be able to give you one of the forms, or can send you a link to a website that will allow you to print one off of the internet.
The exact forms you have to include for your particular courier company to take while making their delivery will depend mostly on exactly what you are shipping. If your goods are worth more than a certain amount there can be additional papers to complete. NAFTA regulated items and goods also have their own forms that must be filled out, but again, this is something that your local delivery company should be able to discuss with you. If your account representative is every unsure of the answer, just ask to speak to the person who specializes in customs and border papers to help you make sure you've filled out everything you need to have ready.