I've experimented with 12 vs. 15 before. Null result. I might try pushing it down to 8 sometime. But, realistically speaking, only if it strikes my fancy some morning.
Ok, I freely admit that I know nothing about selling bingo cards.. But it "feels" to me like that number should be closer to 5. I was really surprised to see that you give away 15 cards for free. You could A/B test this too, right?
Another idea: start high and ratchet down with each subsequent word list?