Take the tubes out and pack them seperately, than put them in the back of the amp. Make sure and pack them really well with bubble wrap.
You should get a box that is slightly oversized for the amp. Wrap the amp in plenty of bubble wrap to the point where it fits snug in the box. Extra protection for the coners would be a good idea as well.
Basically you want the amp very well padded, and packed so it won't shift around in the box.
Make sure you get plenty of insurance, and insist on using a good shipping method that gives delivery confirmation (Xpress Post is good for shipping in Canada). You also might want to add some handling fees to the shipping free because the materials can run you a fair amount of cash.
I have only had one bad online transaction and it was because of poor amp packing. The guy took it to the UPS store, and the idiots only filled the box with peanuts. Not only do they not provide enough protection, but they get all into the amp components and make a mess.