The open-source solution is affixing your credit card (or an "additional cardholder" card) to the back of your phone.
Google Pay, Apple Pay, Samsung Pay are the ones I know of. My bank's app used to be able to do mobile payments but they stopped unfortunately. Banks bristle at the fact there could be open source solutions to anything in the financial sector, it could take away from their upcharging and parasitism.
Security-wise that is significantly worse. Google Pay generates a random card number per transaction and isn't active when the phone is locked.
I keep my credit cards in an NFC blocking sleeves because the passive NFC can't be turned off. Someone could literally bump into you and cause a transaction.
Enough USD can support anything. And for privacy all that phone app stuff doesn't come close. Why degoogle just to throw yourself into yet another spyware app? Anyway the obvious competitor is Apple. Or you could use an ordinary credit card instead of a phone.
I'm not aware of any, but I hope to be proven wrong (edit: maybe PayPal - see below).
AFAIK, NFC payment apps have to be linked to your bank and have some kind of relationship with them. The app doesn't just "spoof" your card over NFC (which would make card cloning terrifyingly easy).
So, any other Google/Apple Pay app would have to be setup similarly and have relationships with banks and card providers/payment processors. I don't know of any third party services that have done that since there's likely a lot of financial regulations around it (just guessing on that part).
I haven't tried that since there's always a line of people, and I'm too embarrassed to cycle through several apps to test if they work at any particular retailer. lol
You may also check to see if your bank's mobile app allows you to setup contactless payments.
But if you're looking for a FOSS solution, I doubt there is one for the reasons mentioned above.
Some number of apps are using pay for more than NFC payment, for ex some ticketing applications from concert organizers don't seem to offer an alternative to use of pay. I was also hoping for support of NFC payment somehow, but had low hopes because I expect there's PCI compliance or whatever.
Because I don't know the backend of the compliance process etc, not an I particularly interested in researching that, figured I'd ask in this community and find at least a couple of knowledgeable answers.
There are card providers like Curve who let you add multiple bank, credit, and store cards to one card, and select which one you want to use through an app. I don't know whether you can use them through your phone separately though, as I only ever tried it through the Google Pay system.
Curve changed their terms a few years ago so that you can only have one payment card without paying a monthly fee though, so I haven't used them for a while. On top of that, changing the selected card through the app was too slow for when I was in a queue and wanted to use a store card and then a payment card.
Ok, thanks very much for this, I'll look into this app.
I'll go on the record acknowledging that inevitably someone is selling my records and habits, but key is knowing the data is siloed across a few different brokers. If someone wants my info, I have to hope adding a bit of a paywall across it will help a bit.