Just as an aside.... I believe that at Safeway and Von's in the US that have gas pumps (possibly the other Safeway owned chains too) you can give them your Canadian Safeway Club card, or phone number that is associated to that card, and get the discount that they offer at the pump. I know that they offer a discount at least at the Las Vegas Von's on Maryland Parkway.

My guess is that the zip code has nothing to do with security and everything to do with marketing - providing data. If the credit card company requires such authorization, then 99999 works best - but since people have entered all sorts of things it looks like this is not the case. If you need a US address for some web sites that normally do not do business with Canada, I found that Toronto, CA 99999 works with most.

For me I always decline to give it to them and of course they still do the purchase. (why lose a customer over a postal code ...) The only store that know my info is the one I maintain an account with.

I have been asked for my 5-digit zip code for shopping at Best Buys in California while I have been asked for the first 3 characters of my postal code when shopping at Safeway in British Columbia. Both firms, after having asked them why, indicated the purpose for creating a marketing survey and having studied marketing myself, it's logical.

Well, technicaly, there are US ZIP codes for Canadian cities, e.g. for Calgary it's, for Toronto it's; the ZIP code for Canada isHere's a complete list (ignore the leading 0 in the 2nd group of numbers and the last digit in the 3rd group of numbers):

So many US stores ask for your zip code nowadays and I can't help but have some fun with it instead of simply saying that I'm Canadian. Years ago I used to use 90210 (from the TV show), then I used 98227 (Bellingham, WA) and since I've moved to YYC I use 59411 (Babb, MT) which is just across the border from Cardston, AB. Don't be surprised if they begin opening a Bed, Bath and Beyond, Pottery Barn and Nordstrom there in the near future.