Quick answer
Costco.com is for US delivery; Costco.ca serves eligible Canadian addresses. The sites have separate pricing and selection. Costco.com accepts Canadian cards and billing addresses for delivery to a US address.
Before ordering
Check eligible destinations on the product listing. Acceptance of a Canadian card does not convert a US order into international delivery. Any separate forwarding arrangement requires confirmation of its terms and cost.
Compare the full cost
Compare the same product and quantity, then add the charges actually shown for the order. Distinguish prepaid charges from any amounts still payable by you. Keep the confirmation and applicable terms.