I'm talking about the slip under the windshield cards, which is what I'm assuming the vendor wants. *shrug*
Why would a vendor want one of those? Individual members created their own (using Vistaprint, for instance) to 'tag' spotted Subarus, inviting them to come discover WSC. There is no standard design and they don't prove anything other than that a WSC fan at one point or another tagged your car.
Most stores that offer a discount requiring proof (student discount, teacher discount, club member discount) are asking for proof of membership to that particular group (student card, teacher certification card, club membership card). WSC doesn't have membership cards, since WSC doesn't offer, deny, renew or revoke membership, per se. We're just an online forum, really. We don't even have a secret handshake.