How can I build this into something bigger?

You have already proven it has interest. Certainly use an affiliate system of some sort. Build a membership. Get on JVZoo and advertise for JV partners. I would become your affiliate and I would promote your affiliate offer. I know many more who would become your affiliate and promote you. Your Army is waiting so take your time in the planning phase because sharing is your fuel and cards are an emotional decision this idea has been proven by Hallmark and others. There was a time about 10 years ago before I began teaching, blogging and creating websites back when I was teaching Computer troubleshooting and helping my students get A+ Certified I had many adult students with diverse backgrounds. So one day a bubbly mature motherly spanish woman came in during class and seated herself at the back of the class. After about 30 minutes of listening she raised her hand and asked "Is this class for beginners? Because I just need to learn how move my business to the computer". So I asked what kind of business is it? "I Sell Greeting Cards" she replied. A few arrogant students chuckled others said out loud what kind of business is that. (This was a new class so no one yet knew the tone of my instruction was based on teamwork and encouragement.) So next I asked how does it work? and she said "Through the mail. I make the cards and send them to my customers who have their own novelty stores and gift shops." Everyone in the class started listening with more intensity as I continued the questions "How many clients do you have?" " Oh I have around nine-thousand. I contact them before during and after Holidays. Look here let me show you some of my new ones. And sure enough she pulled out a Notebook portfolio and I invited the entire class over to where she started pulling out her cards and placing them on the desk for the whole class to see. So I said 'How did you get so many clients?" and she said my friends told their friends and the people who like them want the personal touch. So I took the rest of the day and we talked about newsletters marketing email, etc. She eventually enrolled in the Microsoft Office Course and learned what she needed. However, she was an encouragement when I decided to move into product creation. Scaling sometimes involves learning something new and sometimes it means diversity like taking a working business model in one language then repeating the same model yet in a different language. Twitter, Tumblr, Digg, Pinterest and SEX.com are my top suggestions for marketing