So using that website, I just selected to ship to Canada a flat-rate box with max weight of 20 pounds. Price came to $64.95.
Dimensions of the flat rate box were 13-5/8" x 11-7/8" x 3-3/8" or 11" x 8-1/2" x 5-1/2".
I would expect other sized boxes (i.e., the non-"flat rate box" from the USPS) to be more expensive. Thus, if you really have quite a lot of souvenirs, it might really be more economical to pack it in your luggage instead of shipping.
And keep in mind that the USPS assumes that you will pack everything up yourself and arrive at the post office with everything to go. They will not necessarily give you packing materials, packing tape, scissors, etc.
UPS and DHL ship internationally but would be more expensive.