Yes, Costco is sorta quirky with how the membership works. I believe the membership number is global. It was super easy to renew at a Canada Costco for CAD$120 vs USD$120 (in the US). Do Costco Japan shoppers scan their US costco cards in Japan at check-out?

I am able to have both a costco.com (USA) and costco.ca online account. However costco.ca requires a bank account at a larger Canadian bank for id verification.

I spend most of my time in Vancouver, Canada, but still use my US Costco card without any issues. I receive my 2% dividend check in USD. I assume that when I shop in Canada, the rate is converted back to USD for my 2% rebate check.