This is what it shows as the requirements:
Bring the following documents to the Costco membership counter to pick up your new cards:
Email from Costco (with membership number)
Valid government-issued photo identification
Business license (or three different pieces of business ID)
Resale certificate (if applicable)
Tobacco certificate (if applicable)
We are legally required to keep copies of your business certificates on file.
Does a sole proprietor count as a Business license?