Can you explain how this works?

I live in Canada, shipping is $8 flat rate, and all the taxes and shipping are totalled on the "preview" screen.

I've never paid customs fees in all the years of ordering, so for my situation I know the full price upfront before ordering

This is a problem with shipping with Fedex too. There are a bunch of variable charges that aren't finalized until after the shipment has been delivered. In practice it is rarely and issue and using post paid labels instead of prepaid shipping labels is a better system for most businesses anyways.

Is the order cost variable with digikey because the shipping weight is not known or is there another reason?