Here's the thing though. If you assume that you need to give someone a free card, which I'm all but certain of, it doesn't really matter how easy your app is to use, how many features it has, or anything like that. All those things control is your conversion rate of people who get to the point of sending a free card. (In my case, I was getting about 33% conversion rate here, which I think is pretty great.) So, 1 in 3 people who downloaded the app got all the way to the point of sending a free card.
The catch is who comes back and spends money? It's this conversion rate that determines the fate of the business. You don't have a lot of wiggle room here. You can spam them. You can try to improve the design and fullfillment of the cards so they get positive feedback from the recipient. You can give the recipient a way to notify them they enjoyed the card. You can even get people who receive the cards to pay for credits for them. I did all this. But beyond that, what else can you really do to entice users to come back and spend money? This isn't about improving the flow of the app, the use cases, qr codes, or whatever, it's about taking someone who experienced the product for free to decide to pay for more. It's all about sales.
There aren't many levers here at this point in the funnel, and in my experience this conversion rate I was getting (about 1 out of 8 people) is so low it's fairly hopeless when you consider sunk costs for free cards and not to mention marketing costs. You've spent money to send 8 free cards, which came from 24 installs, and you have one person left who is paying you for more. Ballpark you spent about $15-20 to find this one person. The margin on this person's purchases needs to pay for this plus server costs, employees, support, etc. Depending on your pricing they'll need to send (or buy up front) probably around 10 postcards on average per paid user. You have like no margin of safety here.
You'd really need it to be more like 1 out of 2 or 3 for it to be worth the "lets raise $1m and get to the top of the App Store to reduce marketing costs to zero" case. Also don't forget I was in a highly vertical market where there is a real obvious need (grandparents receiving pictures of grandkids.) Good luck to those guys :) The only way I could see it working is if you are at the top of the app store and you leverage the fact that a large % of your credits go unspent, and you can realize this as a profit. This isn't a business though, it's a ponzi scheme. It might be how Postagram has stayed afloat while they try to make the product solvent.