Well, start by using metric measurements. Canada Post went metric in 1974 and everything else will be approximate.
The metric option is on a dropdown under options, because eBay is an American company.
Then go to canadapost.ca and look up the parameters for parcels in the various services. For example, Letter Post and Light Packet can be no heavier than 500 grams and 2 cm thick. After that, you have to move to Regular Parcel (domestic) or Small Packet (US or International).
A single mm or gram can move your parcel into the next rate. But that rate may allow you to ship two or ten similar items for the same amount.
When you say "irregular" are you referring to tubes or those triangle shipping packets? There is a special price for those.
If you have a parcel that is not a cube, but is a parallelogram, use the longest measurements. Sorry. Canada Post is thinking in terms of how many parcels they can get into the airplane hold or the container. The larger ones are the useful ones.
If you have a specific example, post the dimensions and weight for our opinions.
