From MY experiences with being a buyer and seller, unless the package is SUPER EXPRESS/COURIER between Canada and the US, the tracking number is not going to work.
Canada Post and the USPS do NOT share tracking for STANDARD POSTAGE. Only PREMIUM services.
Mississauga, On. is a major Canada Post facility a Customs facility for mail to the US is processed there.
Mississauga, On. will be the LAST record of a STANDARD POSTAGE package/letter/etc. until it arrives at your door.