I looked at the hat and at the Archie Comics (or are they Teenage Mutant Ninja Turtles-- confusing title).
Both had multiple shipping methods showing when I used 90210.
I wonder if on some of the smaller parcels your measurements are hitting the small disconnect between the Canada Post and USPS sizing. If so, try adding a centimeter to your measurements to make your CP package big enough for the USPS dimensions. Don't worry this does not change the price of shipping.
I assume you are using metric measurements? Canada Post went metric in 1974 and everything else is approximate. The SYI form shows Imperial measures because eBay is a US company.
Might I suggest that you offer fewer shipping choices to your buyers? A service that is Delivery Confirmed is all you need. If the value of the lot is over $250 a service with Signature Confirmation of Delivery is what you need. Don't require too much thinking. Impulse buyers don't want to think too much. They want the rush of buying. Make it easy.
