Weigh and measure the parcel.
Decide on which AIR MAIL service best fits your comfort zone.
Enter the information here;
http://www.canadapost.ca/cpotools/apps/far/business/findARate?execution=e1s1
Send the buyer a quote.
If he objects or asks for cheaper surface shipping , offer a Mutual Cancellation if he sounds polite and friendly, or open an Unpaid Item Dispute if he is at all bullying or belligerent.