coolcrab, I appreciate and love your out-of-the-box thinking with innovative ways of storing data on a calculator. However, there are a number of issues with this project, at least as far as usage for sensitive data goes (credit cards).
1. A calculator is generally not capable of encrypting the information you'd be providing in a secure enough way to be industry-acceptable (keep in mind that if you are designing an app like this you must meet industry standards or your app will not be approved for commercial use). Similarly, if a calculator can decrypt the data fast enough for this project to be feasible, a computer will make short work of it.
2. As a few other people have stated, security-through-obscurity is not the worst thing when used with a conjunction of other security methods, but when it's essentially the main security method, it's horrible. Keep in mind that people in general will not trust your program to keep their data safe unless they know that the encryption in secure, and in many cases, you will not become an approved digital-pay system unless you REVEAL your encryption methods to a gov't committee and it's verified to be industry standard. I mean, you could say "I'm not revealing my encryption methods to keep your data safe", and store them in plain text files and no-one would be the wiser. It's actually beneficial to be able to say "I'm using an industry-standard 512-bit encryption" so people know (or can research) that their data is actually safe.
3. Lastly, and most importantly, if you put out an app that stores user data and that data becomes compromised and results in information theft, YOU are legally liable. This is the part that's most important to fully understand the gravity of, and generally makes endeavours like this not worth the risk unless you plan to profit off this project, which would also make it harder to sell this. If your app's security is ineffective to stop someone from stealing a user's data you could be sued for a LOT of money, even if you don't sell this program.
Just bear these things in mind