Love the USPS. The tracking numbers assigned by USPS also work after the package enters the Canada Poste system. Any brokerage fees charged by Canada Poste are minimal. USPS also seems to be relatively fast.
FEDEX, I have only used FEDEX Ground, they paid the duties and Canadian taxes and then billed me after the fact for them. For the record there are no import duties on amateur radio equipment but we do have to pay either the GST or HST depending upon the province you live in.
UPS, stay away from them, they charge extremely high brokerage fees. I know that some people will cancel an order if the vendor suggests UPS.
As for US postal money orders, they should be negotiable at any chartered bank in Canada.
Paul