Maybe a CPA offer that has broad consumer appeal.

I would advertise "Get a free ad in this mailer"

Then, when they call, you know they're interested in your audience... of course you don't actually have any free ads left, but now that you have them on the phone, I bet you could find a way to discount it...

I would guess you already have a spot for "see your ad here" somewhere on the card? If not - Bob Ross just got rushed to the hospital with chest pains! Maybe an ADT pay per call#?

Off the top of my head Direct TV, ADT, CPA, Charity/Non Profit, do a barter with a business you need something from. Promote your other website/businesses/services.

Negotiate with a small biz you believe in and ask for a commission on the back end. Find a local politician or official up for election and sell them a spot. Write up a blurb about what your card is all about and how to be on the next one. Post a photo of a military family you support. Put a quick article about a local event you may be involved in. Find a local artist or musician who might want the exposure. If you are a fan of the arts, donate itto a museum or symphony and class up the place a little while building good will and exposure at the same time. You might turn them into clients later.

I would create an optin page and try in a way (provide something for free etc.) to get those recipients subscribe to my newsletter by displaying my URL and a call to action in that spare space. Even a 1% conversion rate would result in 100 people in your list that you could start marketing via email as well.

Maybe Dating , PPS & PPL. Depends where its from. Could Test bot PPL & PPS to see where EPC's sit higher