That's unusual, I sell on ebay.ca and receive Paypal payments fine, I've never been asked for a business number.
11 cm x 7 cm x 2 cm and about 1-2 grams.
18.81 Dollars Canadian. To go ANYwhere in Canada... even within my own province.
Are you looking up the rates on canadapost.ca? Make sure you put the measurements in under "large envelope" rather than "parcel". I'm getting $2.03 across the country for something that size and weight.