For some of my bigger items that go via Parcel (XPost, Expedited, etc.) I use calculated shipping. For the smaller things that go via lettermail, I use a flat cost since I know for sure how much the cost would be regardless of where it's going.
Sign up for a Venture One card, you get 5% of most parcel services with Canada Post. Or, you could ship via Paypal Shipping, which gives you ~15%-18% of most parcel services with Canada Post.
Keep in mind ebay takes a percentage of what you charge for shipping.