Shopping in Buffalo USA

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Hi,

I recently submitted papers to sponsor my wife within canada. She has a visitor status until January 2008. Canada does not need any visa for her country of origin, so the question is if we could go shopping to USA and come back without any issues at the border?

When you have a number for the process, would it appear in her or my passport display at the border/airport? Would it be difficult to re-enter Canada?