Are Canadian Costco cards valid in US?

Yes

Yes, the Costco MasterCard now can be used in the US. Previously you were not able due to contractual obligations.

I would say yes, my US card is valid in Mexican Costco's. So I see no difference with a Canadian card in the US

Call Costco. They can tell you.

I'm not sure, but I would assume so. Costco is Costco. Good luck

Yes, cotsco is international.

I believe they are International!

Wow.. Don't know

Yup