Your invoice, credit card (to pay), proof of who you are (if item is in the name of your business, then some business ID). If coming in tax exempt (PST) for resale, then your PST Vendor Permit bearing your PST number.
Also the Bill of Lading info and any Customs Numbers given to yout omake Customs officials' jobs that much easier.
Wear a Happy Face and be pleasant and you will get through alot faster.
Malcolm