Two codes that give better prices than the Kellwood code:
Costco.ca: (Both give same discounts - I don't know if you have to show your membership card)
Regular member: AWD #:C857501
Executive member: AWD #:C857500
Aeroplan: C682300 (also gives you extra Aeroplan points)
Example:
Avis Las Vegas Airport, Intermediate car, 5-day rental:
Costco.ca: $141.63
Aeroplan: $145.64 (extra Aeroplan points worth about $4)
Kellwood (G417900): $184.49
Granted this is a very specific example related to my current search, but they worked for me.