FopenP Calculate Shopping Cost

I created an app that lets you calculate how much you must give to the cashier when you have a certain amount in your credit card and you want pay the remaining with cash. For example, if I have $50 in my credit card and I have an $80 shopping, the app will indicate that I need to pay $30 in cash.
It's useful when you have a gift card or when you want to consume (almost) all the money in your credit card.
I've created two versions: a native one for Android (published on Google™ Play Store, seekable with the "FopenP" text) and a "progressive web app (PWA)" version, that is a web site installable in your smartphone with an its own icon (if you use the Google Chrome browser). I haven't tested with Safari but it should work there as well.
If you want to try the progressive app, please navigate this link: https://fopenp.github.io/calculate-shopping-cost-pwa/
The PWA is not published here because at the moment this hosting doesn't provide HTTPS (unless you do a certain procedure once every three months).