Or do they send those to all Costco members?
Does it say anywhere on the mail offering that your "pre-approved" or "pre-qualified"? if not, or actually even if it does, you can go to the AMEX website to see any "pre-approved" offers you have
I don't think it's a standard Costco member mailing unless they only target specific demographics of thier membership. I'm pretty sure I'm based on the odd looks I got when I signed up most college students aren't the primary card holder on the membership account. If you want to apply for it thought I've heard it's faster to do it in the warehouse because they can verify your membership before sending your application in to AMEX.