I use the bubble wrap envelopes for polys as well... You'll get some cashiers at Canada Post who don't want to let it go through though so be careful. It's $12 to send a bubble mailer as a parcel.
I've shipped quite a few larger items now and this is what I do...
I keep all the boxes, paper, bubbles and air packets from my purchases, some day I'll run out but it's served me well so far. If you're trying to source boxes, try Indigo or TRU. They just recycle them anyways. I've bought giant boxes of bubble wrap from UHaul as well for pretty cheap. For a couple overseas sales where I was shipping about 12kg (well, 4 x 2.93kg) of heavy sets in a lot, I've purchased really strong boxes from UHaul as well.
For packing, first make sure the box isn't a piece of garbage. Heavy sets need good boxes. I always try to ship my sets flat since most cardboard boxes are built that way for strength - some exceptions of course, be cognizant of the flute direction and you're good to go.. It also puts lets outward pressure on the set itself as it bounces around on the delivery trucks. I make sure the box is always a little larger width/length to prevent edge crush. Height doesn't matter as much if you have a good box!
I wrap the set in brown paper or foam paper (I have a box full of 4' x 3' sheets I had left from a home reno). I then fill the voids with packing paper or air packets. Don't fill them too tight, just enough so the box doesn't jiggle. If the box is too tall, throw some paper, bubbles or air packets on top. If it's a box I'm reusing and worried about the side strength, I'll often sacrifice another box to double up the cardboard around the outside edge to get double the strength.
Don't be cheap on the tape to seal it up. Tape is cheap. I tend to over pack even the smallest sets... not worth the risk. It's not magic, just don't do what TRU, Sears, and Walmart often do (slap a label on the box, or put it into a box three times the size with 2 air packets) and you're good to go.