Why do you have to do merch fulfillment on a daily basis? Just inform people before signup that you only send out membership cards on the first of the month and if they sign up at any other time they'll have to wait until the first of the next month to get their card sent in the mail.

Alternatively, they could show up at the nightclub in person and bring their phone with proof of purchase and the bouncer could hand them a membership card and cross their name off a list.