These postage calculators might help:

http://postcalc.usps.gov/

http://payments.ebay.com/ws/eBayISAPI.dll?EmitSellerShippingCalculator

You have to know the type of mail piece you are sending and the weight. Many people consider padded/bubble envelopes to be first class PARCELS.

Anyway, from US to Canada for first class parcel: not over:
1 ounce $3.00
2 ounce $3.15
3 ounce $3.30

Robert