I go to Canada fishing once per year. I've never seen a place that won't accept US currency in Canada. For the most part, the US dollar has been worth more than the Canadian dollar.

But I don't follow mgburg's reasoning. Say right now the Canadian dollar is worth $0.80 in the US. If I am a bank and take Canadian money for exchange and I only give you $0.75 on the Canadian dollar I just made money. That is how the exchange stores work up on the border. They make money on both transactions, US to Canadian and Canadian to US. Most stores in the US just don't want to deal with the hassle since it is so rare anyone pays with another currency.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD