Here are some quick images I threw together based on some of the feedback. Don't pay that much attention to the actual "adventure" (plot, railroad, etc.) but more on how it's laid out.
I did this with Twilight Fables. While lots of people were interested in the book. No one wanted the cards. Maybe 2 people over the past 2 years. I had a physical cardstock option, and a print-your own PDF option exactly as you describe.
How do I like to earn it? By the bucket loads.
In all seriousness, I did AD&D style XP awards for decades. But as I get older and game time is less frequent, I want to manage fewer things, so I tend to lean toward milestone now.